Multi-Channel Laser Pump Source for Optical Amplifiers

ABSTRACT

An optical assembly, such as a multiple output diode laser pump source for EDFAs, is formed by pressing an optical array emitter chip against a standoff structure protruding from a submount such that the emitter chip deforms to match the curvature of the standoff structure. An IO chip is also juxtaposed against the standoff structure such that its optical receivers can receive optical energy from the emitter chip. The IO chip can provide various optical functions, and then provide an optical array output for coupling into an optical fiber array. The standoff structure preferably contacts the emitter chip over an aggregate contact area much smaller than the area by which the emitter chip overlaps the submount. The materials used for bonding the emitter chip and the IO chip to the submount are disposed in the recesses between standoffs and not on the contact surfaces of the standoff structure.

PRIORITY INFORMATION

This application is a divisional of U.S. application Ser. No. 10/898,537filed 23 Jul. 2004 (which is set to issue on 26 Jun. 2007 as U.S. Pat.No. 7,235,150), titled “Multi-Channel Laser Pump Source for OpticalAmplifiers,” which application is a continuation of U.S. applicationSer. No. 10/616,008 filed 09 Jul. 2003, titled “Multi-Channel Laser PumpSource for Optical Amplifiers” (now abandoned), which application is acontinuation of U.S. application Ser. No. 09/784,687 filed 14 Feb. 2001,titled “Multi-Channel Laser Pump Source for Optical Amplifiers” (nowabandoned), all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor diode laser based pumpsources, and more specifically to techniques for construction of planarmulti-channel diode laser pump sources.

2. Description of Related Art

Optical amplifiers are an accepted part of long-haul telecommunicationssystems. They are used to amplify signals after optical fiberpropagation losses over long transmission distances typical of such asthe nation-wide networks. A typical system uses a plurality of Erbiumdoped fiber amplifiers (EDFAs) pumped by semiconductor diode lasers.Semiconductor diode laser pump sources for EDFAs typically operate atwavelengths of 980 nanometers (nm) or 1480 nm. The EDFA is capable ofamplifying wavelengths over a wide bandwidth with a gain spectrum thatpeaks at about 1530 nm and that typically extends to 1570 nm, or 1620 nmin advanced configurations. Usable output or optical gain is achievableover this 40 to 90 nm region. This wide bandwidth provides theopportunity for the optical signal to be carried on a large number ofwavelength channels that can be independently and all-opticallyamplified by an EDFA. This technique of wavelength division multiplexing(WDM) is currently driving the expansion of modern telecommunications.

The EDFA has been available for about 10 years, during which time theperformance of the device has increased markedly, benefitting inparticular from improved performance of semiconductor diode laser pumpsources. However, this improved performance is typically accompanied byincreased cost. The increased cost is readily tolerated in the highvalue, relatively low fiber count long haul system because each fiber isable to carry many data channels via WDM, each data channel beingamplified simultaneously within a single EDFA. Thus the cost of the highperformance EDFA is shared amongst many revenue generating data streamsand subscribers.

To achieve continued expansion of data carrying bandwidth to the officeand home, the fiber optic transmission system must be extended from thepoint-to-point long-haul network to Metropolitan (Metro) and accessnetworks. The more diffuse nature of the Metro network, and the need toservice users on a more individual basis means that less data is carriedonto a single fiber, generally causing proposed Metro networks to becharacterized as having fewer data channels per fiber at lowertransmission rates and more individual fiber transmission paths andshorter span lengths than long haul systems. The lower data channelcount per fiber means less revenue per fiber, and the increased numberof separate fibers and decreased span length means increased componentnumbers. The combination of these two factors leads to a requirement oflower cost components to enable profitable Metro network implementation.

EDFAs play a key role in Metro networks, just as they do in thelong-haul backbone. The use of EDFAs enable longer ring or meshpropagation distances within the network, and also enable the use oflossy all-optical components such as wavelength demultiplexers andmultiplexers, or optical cross connects without the need for costlydetection, electrical regeneration, and reemission/modulation of thedata signals. Thus a lower cost implementation of the current EDFA foundin long-haul networks is required to drive the installation andcommissioning of Metro networks.

A typical long-haul network EDFA is composed of a number of subsystemsor components including one or more erbium doped fiber sections, opticalisolators to eliminate back reflection, and one or more semiconductorlaser diode pumps with their associated wavelength couplers to combinethem with the 1550 nm data signal on the network fiber. A significantproportion of the overall cost of the amplifier results from thesemiconductor diode laser pumps, which typically cost many thousands ofdollars each. Thus, a lower cost implementation of the diode laser pumpsource would enable lower cost EDFAs for application in Metro networks.

Currently, two main types of diode laser pump sources exist: those thatoperate at 980 nm and those that operate at 1480 nm. These twowavelengths are absorbed quite efficiently by the erbium ions in thefiber core and offer different performance characteristics for theoverall amplifier system. Pumping at 980 nm is usually chosen forpre-amplifiers where low noise amplification is important, as the 980 nmpumping may lead to a more complete population inversion of the emittingerbium state and to a correspondingly lower amplifier noise figure ascompared to 1480 nm pumping. Diode lasers operating at 1480 nm are oftenchosen for high output power amplifiers as the optical-opticalconversion efficiency is higher and the dollar cost per mW of outputpower from the diode laser is generally lower than for 980 nm diodelasers.

Prior art semiconductor diode laser pump sources operating at 980 nm (avery similar device configuration is used at 1480 nm, simply utilizing adifferent semiconductor material system to generate the differentwavelengths) generally consist of a number of individual components,shown symbolically in FIG. 1. A diode laser chip 105 is soldered to asubmount 110 to provide thermal heatsinking and electrical connection.Each diode laser chip 105 has a single active laser region that iscapable of generating and emitting several hundred mW of output power ina single transverse optical mode. Submount 110 is positioned inside abutterfly package 115 that has the capability of achieving a hermeticseal on final closure. A single-mode optical fiber 120 is fed through aferrule and opening in butterfly package 115 and then brought intoalignment with the output aperture of diode laser chip 105. To achieve ahigh coupling efficiency between diode laser chip 105 and single-modeoptical fiber 120, a lens or chisel shaped tip may be formed on the endface of fiber 120 so that the rapidly diverging optical mode of diodelaser chip 105 is efficiently converted into the relatively much largerand slower diverging mode of fiber 120.

In addition to the need for the lensed or chisel ended fiber 120, thereare also very tight constraints placed on the positioning of the tip offiber 120 relative to the emitting aperture of diode laser chip 105. Infact, it is necessary to control the position of fiber 120 to sub-micronaccuracy to achieve optimum coupling. This precise control is typicallyachieved by holding fiber 120 via a computer controlled multi-axismicropositioner. Diode laser chip 105 is energized to generate outputlight, and the output from fiber 120 is monitored using a photodiode orpower meter. The micropositioners then move fiber 120 to optimize formaximum signal transmitted therethrough, after which fiber 120 is fixedin position, typically by laser welding of a metallized fiber ferrule125 to a holder clip 130 mounted to the package or submount 110. Oftenit is necessary to tweak the alignment of fiber 120 after initial fixingwith further laser-assisted or mechanical bending of holder clip 120.

The fully active fiber alignment process described above is bothcumbersome and slow, and although it can result in remarkably goodcoupling efficiency between the diode laser and fiber (in excess of60%), it does not lend itself well to high volume, high yield and lowcost manufacturing. It is this fully active alignment step that accountsfor a significant portion of the cost of constructing a diode laser pumpsource for an EDFA.

In the prior art, attempts have also been made to constructmulti-channel integrated laser arrays and to align them with integratedoptoelectronic chips or directly with fiber arrays. The use of aself-aligned solder assembly with mechanical stops and misaligned solderjoints is reported to provide three-dimensional passive alignmentbetween the laser diode axis of each diode and a corresponding opticalaxis of an optical fiber with lateral misalignment of ±2 microns andvertical misalignment of ±0.75 microns, with coupling losses of about 4dB per channel reported for a 4 diode array.

Such prior art techniques do not provide the level of precision obtainedusing the active fiber alignment described above, and which isconventionally required to achieve efficient coupling between a diodelaser and an optical fiber. Difficulties include height variation (thatis spacing between the diode laser chip and the substrate) across thelateral array dimension due to solder thickness variations or bondingpressure variations. Thus, co-planarity of the bonding surfaces isdifficult to achieve to the degree desired for very efficient coupling(typically about 0.2 microns over 10 mm for a high channel count laserdiode array).

Bowing of the diode laser chip also gives rise to misalignments acrossan array of emitters. Fabrication of the diode laser structure usingepitaxial growth and planar surface lithography often results in a laserarray chip with a residual bow or warpage.

In addition, misalignment can also be caused by particles trappedbetween the diode array and the mounting surface. Keeping large planarsurfaces free of particles is difficult. Even a single sub-micronparticle is sufficient to cause severe misalignment along a large array.In one prior art approach, the presence of foreign particles isaccommodated by supporting the laser array on a pair of standoffs, abovethe large substrate area, one standoff at each end of the array. Thismay help to minimize the effect of foreign particles, but does notaddress the absolute positioning of the multiple emitters across thearray, and cannot alleviate misalignment arising from laser arraycurvature due to warped or bowed wafers.

In view of the problems associated with prior art techniques formanufacturing pump sources, it is an object of this invention to providea semiconductor laser array pump source for optical amplifiers which maybe manufactured relatively easily and inexpensively, and which enablesprecise optical alignment of components even in the presence of foreignparticles and/or component warpage or bowing.

SUMMARY OF THE INVENTION

Roughly described, a multiple output diode laser pump source ismanufactured by an array scaleable optical alignment process, which canachieve simultaneous highly efficient coupling between each of theemitters in a laser diode array and a respective receiving waveguidearray. The waveguide array may be fabricated (without limitation) in alithium niobate, glass substrate, or other integrated optics chip. Thesimultaneous coupling of all of the emitter/waveguide pairsadvantageously reduces the number of manufacturing steps, therebyallowing the pump source to be manufactured more rapidly andinexpensively.

According to one aspect of the invention, the simultaneous arrayscalable alignment process utilizes a standoff structure on a submountto define a reference surface for mounting of a laser array and areceiving integrated optic waveguide chip. Mounting both chips to thesingle reference surface permits optical alignment in the most criticaldimension, perpendicular to the plane of the arrays, to be performedpassively, leaving active alignment required only in the transverse andlongitudinal dimensions and the yaw axis.

In addition, the standoff structure also can overcome other problems inthe prior art by, for example, eliminating the effect of solderdeposition thickness on emitter position, lessening the impact offoreign particulate defects by decreasing the contact area between thearrays and the submount, and alleviating the effects of non-uniformbonding pressure and chip bow or warp by distributing the referencesurface across the entire array and therefore referencing the positionsrelative to the submount of all emitters directly to the same referencesurface.

The invention will be better understood upon reference to the followingdetailed description in connection with the accompanying drawings:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a symbolic diagram of a prior art diode laser pump source.

FIG. 2 is a symbolic diagram of a laser array pump source according toan embodiment of the present invention, shown without a protectivepackage.

FIG. 3 is a cross-sectional view taken along line A-A in FIG. 2.

FIG. 4 is a fragmentary symbolic diagram of a submount according to anembodiment of this invention.

FIG. 5 is a symbolic diagram of a second embodiment of the invention,showing in particular an IO waveguide array chip with distributed Braggreflector structures (DBRs) for wavelength stabilization and waveguidetapers for mode size conversion.

FIG. 6 is a symbolic diagram of fused-fiber-optic directional couplersarranged to combine light output at different wavelengths from severalemitters of a laser array.

FIG. 7 is a symbolic diagram of waveguide directional couplers formed onthe IO waveguide chip and arranged to combine the light output atdifferent wavelengths from several emitters of a laser array.

FIG. 8 is a symbolic diagram of optical fiber polarization multiplexersarranged to combine the light output from pairs of emitters of a laserarray.

FIG. 9 is a block diagram illustrating detector array locations foroutput monitoring and power stabilization.

FIG. 10 is a symbolic diagram, show in plan view, of an integrated powermonitoring detector array on an IO waveguide array chip.

FIG. 11 is a cross-sectional view taken along line A-A of FIG. 10showing locations of distributed Bragg reflector and optical detectorchip (two alternatives) for power monitoring.

FIG. 12 is a symbolic diagram, shown in plan view, of an integratedpower monitoring detector array using waveguide directional couplers.

FIG. 13 is a symbolic diagram showing a redundancy-providing arraydesign in plan view.

FIG. 14 is a symbolic diagram, shown in plan view, of an embodimentproviding for redundancy using an optical fiber switch matrix.

FIG. 15 is a symbolic diagram, shown in plan view, of an embodimentproviding for redundancy using a waveguide switch matrix on an IOwaveguide array chip.

FIG. 16 is a schematic diagram of a passive network of optical fibercouplers and splitters for power sharing among the optical fiber outputsof an array source.

FIG. 17 is a symbolic diagram, shown in plan view, of an optical fiberarray disposed in v-grooves on a submount.

FIG. 18 a is a symbolic diagram of a laser array pump source using twosubmounts with a standoff structure.

FIG. 18 b is a symbolic diagram, shown in elevated end and perspectiveviews, of an optical fiber array assembled on a silicon substrateadapted with v-grooves.

FIG. 18 c is a symbolic diagram of an IO waveguide array chip coupled toa v-groove optical fiber array, on a submount with a standoff structureand relief slots.

FIG. 19 a is a symbolic diagram, shown in plan and elevated side views,of an alternative embodiment of the invention wherein a laser array iscoupled directly to an optical fiber array.

FIG. 19 b is a symbolic diagram showing a perspective view of thealternative embodiment of the invention depicted by FIG. 19 a.

FIG. 20 a is a symbolic diagram showing a waveguide directional couplerfor wavelength multiplexing, and distributed Bragg reflectors (DBRs) forwavelength stabilization positioned before the directional coupler,arranged on an IO array chip.

FIG.20 b is a symbolic diagram showing a waveguide directional couplerfor wavelength multiplexing, and DBRs for wavelength stabilizationpositioned in series after the directional coupler, arranged on an IOarray chip.

FIG. 20 c is a symbolic diagram showing a waveguide directional couplerfor wavelength multiplexing, and DBRs for wavelength stabilizationpositioned in series after the directional coupler, with one DBR on theIO array chip and another DBR on an output fiber.

FIG. 20 d is a symbolic diagram showing a waveguide directional couplerfor wavelength multiplexing, and DBRs for wavelength stabilizationpositioned in series on the output fiber.

FIG. 21 is a block diagram of multiple EDFAs pumped by a single laserarray pump source of the invention.

FIG. 22 is a fragmentary perspective view of the FIG. 2 embodiment,showing in particular the relationship between corresponding ports ofthe laser array and IO waveguide chip.

DETAILED DESCRIPTION

The invention will now be described in reference to various non-limitingembodiments thereof. It should be noted that individual elements orfeatures of different embodiments described below may be combined invarious permutations to produce a laser pump source having a set ofdesired physical and/or operational characteristics, and that suchcombinations are within the scope of the present invention.

FIG. 2 illustrates an embodiment of a semiconductor diode laser pumpsource 200 in accordance with aspects of the invention. Pump source 200is shown to include several major components: a submount 205, a diodelaser array (hereinafter “laser array”) 210, an integrated optic (IO)waveguide chip 215 and an optical fiber array 220. As used herein, anoptical “array” is a device that includes two or more optical ports. Thesubmount 205, laser array 210 and IO chip 215 are shown in greaterdetail in FIG. 22, which illustrates in particular opposing edges 250and 255 of laser array 210 and IO waveguide chip 215. Laser array 210and IO waveguide chip 215 are each provided with a plurality of opticalports 280 and 285 arranged along a respective subject edge 250 or 255.The optical ports may take the form of waveguide inputs and outputs,positioned such that the subject edges 250 and 255 of laser array 210and IO waveguide chip 215 are substantially perpendicular to the opticalaxes 290 of the waveguide inputs and outputs, with the lateral spacing240 in the horizontal dimension parallel to the subject edges beingsubstantially the same in both components, at least at the subjectedges. In this manner, the optical ports of laser array 210 and theoptical ports of IO waveguide chip 215 can communicate with each otherif they are arranged in such a way that a sufficient fraction of theoptical energy emitted from one of the ports is captured by the otheroptical port. A condition of perfect alignment between correspondingports, wherein the optical axes of the pair of corresponding ports arecoincident, is not required. Rather, two ports “can communicate” witheach other even if their optical axes are spatially or angularly offset,provided that such spatial and/or angular offset does not exceed adesired tolerance. Corresponding ports also “can communicate” with eachother if the ports are aligned (or misaligned by no more than a desiredtolerance) with an optical path that includes an optical redirector(such as a reflector, refractor, or re-emitter) positioned intermediatein the optical path between the ports. Similarly, as used herein, oneoptical port “can receive” optical energy from another optical port ifthe two optical ports “can communicate” with each other.

As is known in the art, laser array 210 is configured to emit lightresponsive to application of electrical signals thereto. The term“light” as used herein is not limited to visible light, but ratherincludes any optical energy having a wavelength or range of wavelengthssuitable for a specified application. Light emitted by laser array 210is coupled into the receiving end of IO waveguide chip 215, where awaveguide taper array may be used to convert the mode emitted by laserarray 210 to that of a single mode optical fiber. IO waveguide chip 215may also include other functionalities as described below, such asincorporation of distributed Bragg reflectors (DBRs) for wavelengthstabilization, directional couplers for wavelength multiplexing, andswitch networks for redundancy. At the output end of IO waveguide chip215, the transmitted and mode-converted light is coupled into an opticalfiber array 220 to be conveyed, for example, to the amplificationregions of EDFAs. In accordance with an aspect of the present invention,assembly of the several components to form the pump source 200 isadvantageously effected by simultaneous optical alignment of theemitters of laser array 210 to corresponding receiver waveguidesfabricated in IO waveguide chip 215, followed by (or performedconcurrently with) the alignment of optical fiber array 220 to theoutput end of IO waveguide chip 215. As is discussed in further detailhereinbelow, simultaneous alignment of the laser emitters to thecorresponding receiving waveguides may be achieved by utilization of asubmount 205 having a standoff structure 222 to which laser array 210and IO waveguide chip 215 are attached.

As illustrated in FIG. 2, standoff structure 222 may comprise a set ofdiscrete parallel standoffs 230 fabricated on and protruding verticallyupward from a first major surface 227 of submount 205. Standoffs 230terminate at their upper end in upper surfaces, at least a portion ofwhich contact corresponding surfaces of laser array 210 and IO waveguidechip 215. As used herein, the term “vertical” is intended to meansubstantially perpendicular to the major planes of submount 205, laserarray 210 and IO waveguide chip 215. Areas of first major surface 227lying between discrete standoffs 230 define an array of wells 229 orrecessed regions of a depth suitable for receiving solder balls and/oradhesive (for example a glue, epoxy or other such bonding agent). Thewells 229 may also carry circuit metallization traces and contact padsfor electrical interconnects.

Standoff structure 222 may be fabricated on first major surface 227 ofsubmount 205 by photolithographic and selective etching processes.Alternatively, standoff structure 222 may be fabricated by laserablation, by depositing a layer of material and defining standoffstructure 222 by photolithography and a solvent or etchant, bypositioning and affixing material preformed in a predetermined thicknessand shape, or by a combination of the foregoing techniques. Standoffstructure 222 may alternatively be formed on the appropriate majorsurface of laser array 210 or of integrated optic waveguide chip 215, orpartially on laser diode array 210 or the integrated optic waveguidechip 215, and partially on submount 205. As shown, discrete standoffs orribs 230 extend primarily longitudinally on submount 205 (thelongitudinal axis being in the horizontal plane of submount 205 andoriented substantially perpendicular to the subject edges of laser array210 and IO waveguide chip 215).

The function of standoff structure 222 is made clearer with reference toFIG. 3, which represents a cross-sectional view taken along the line A-Aof FIG. 2, and with further reference to FIG. 4, which depicts afragmentary perspective view of submount 205 in the absence of laserarray 210 and IO waveguide chip 215. Ribs 230 contact laser array at aplurality of contact portions 232. Contact portions 232 collectivelydefine a reference surface 305. Reference surface 305 will typically besubstantially planar, although some implementations of the invention mayutilize standoff structures which define a curved reference surface(i.e., a reference surface having a finite radius of curvature along thelateral and/or longitudinal axes). It will be appreciated that someportions of the standoff structure may be shorter than others, eitherintentionally or unintentionally, and those will not form contactportions with laser array 210 and do not count in the definition of thereference surface 305.

It should be noted that standoff structure 222 may be formed in avariety of geometries and is not limited to the parallel discrete ribdesign depicted in FIGS. 2-4. For example, standoff structure 222 may beformed as a continuous serpentine-shaped structure, or as a comb-typestructure having a plurality of teeth joined at one end by a commonspine. Irrespective of the exact shape which standoff structure 222takes, it is preferable that at least three of contact portions 232occur consecutively along a substantially straight line parallel andclose to the subject edge of the chip being supported (i.e., laser array210 or IO waveguide chip 215), and that those three contact portions 232are mutually isolated from each other along that line in referencesurface 305. Note that contact portions 232 may, however, connect witheach other at locations not on such line, and therefore may not beentirely isolated from each other; but they should be mutually isolatedfrom each other at least along some line in the reference surface 305parallel to and in close proximity to the subject edge.

It is to be appreciated that, in the lateral dimension along the subjectedge, contact portions 232 should be sufficient in number, spacing andextent to effectively define the curvature of the subject edge for aspecified application. For many applications, the subject edge will havean infinite radius of curvature, i.e., will be linear. In thelongitudinal dimension, contact portions 232 should be sufficient innumber, spacing, extent and proximity to the subject edge that theycontrol the curvature of the subject edge adequately for the desiredapplication. It is preferable that at least one contact portion includedunder the optical component 210, 215 is sufficiently close to thesubject edge to define the vertical angles of the optical axes of eachof the optical ports adequately for the desired application.

It should be appreciated that the present invention does not require auniform height for all elements of standoff structure 222. In someembodiments, required curvatures and optical axes' angles are bestachieved with non-uniform standoff structure heights. For example, astandoff structure may comprise a stepped standoff structure comprisinga first set of standoffs having a first height, and a second set ofstandoffs having a second height different from the first height. Thefirst set of standoffs may then be utilized to support laser array 210,and the second set of standoffs may be utilized to support 10 waveguidechip 215. In this case, it should be noted that the two referencesurfaces defined by the first and second sets of standoffs are notco-planar. The stepped standoff structure design may be useful inachieving vertical alignment of optical ports when laser array 210 andIO waveguide chip 215 have substantially different thicknesses. Inaddition, the stepped standoff structure may provide a physical stopwhich enables precise longitudinal positioning of IO chip 215, thusfacilitating butt coupling to laser array 210. It will be apparent toone skilled in the art that protrusions functioning as physical stopsmay be incorporated, where desired, into standoff structures having anotherwise uniform height.

It should be further noted that contact portions 232 of the standoffstructure 222 that are under one optical component, for example laserarray 210, can be either continuous with or discontinuous (distinct)from corresponding contact portions located under the second opticalarray component, for example IO waveguide chip 215. Standoff structure222 should be substantially rigid, at least along the longitudinal axisthereof, to achieve proper alignment of the optical ports of laser array210 and IO waveguide chip 215. In the vertical dimension, standoffstructures should have sufficient rigidity to withstand the pressuresexerted by the chuck pressing against it, as described elsewhere herein.If the pressures are exerted at elevated temperatures, which might bethe case if certain bonding agents such as solder are being used, thenthe vertical rigidity of the standoff structure should be maintainedeven at the elevated temperature. In this sense, a solder bump which isliquified during mounting does not qualify as part of a standoffstructure.

As used herein, the terms “top,” “bottom,” “lower” and “upper and thelike are used solely for convenience in referring to particular levels.The levels they refer to are not intended to change if the structure isturned upside down or tilted.

It is further noted that the output light emerging from the emitters oflaser array 210 diverges rapidly. The rate of this divergence isinversely related to the dimension of the optical mode within thewaveguide cavity. For optimally efficient operation of the laser diodeemitters, the multiple layers composing the gain (or diode junction) andwaveguiding regions in the semiconductor chip are constructed such thatthe optical mode is tightly confined, with typical optical modediameters of 1 μm in the vertical dimension (perpendicular to the planeof the diode laser junction) and 3 μm in the lateral dimensiontransverse to the optical axis of the diode laser waveguide. Thus thelaser diode output diverges faster in the vertical dimensionperpendicular to the plane of the laser diode junction, compared to thedivergence rate in the lateral dimension. This small vertical dimensionand the corresponding rapid divergence makes the vertical alignmentbetween two such waveguides the most critical, i.e. having the smallestallowable misalignment tolerance for efficient coupling.

To achieve accurate alignment between the emitters of laser array 210and the corresponding waveguides of IO waveguide chip 215, standoffstructure 222 defines a reference surface 305, as illustrated by anddescribed earlier in connection with FIG. 3. This reference surface 305is used to accurately align the optical axes of the emitters of array210 and the receiving waveguides of the integrated optical waveguidechip 215 in the critical vertical dimension. This approach preferablyemploys a flip-chip bonding technique, which involves bonding componentsof pump source 200 with the active surface (also variously referred toas the “top”, “upper” or “circuit” surface) of the component facingsubmount 205. According to typical construction methods of IO devices(such as laser array 210 or IO waveguide chip 215), several devices of agiven type are typically fabricated simultaneously by photolithographicand planar processing techniques on a single wafer, which is thenseparated into identical smaller units referred to as chips, components,or devices. As a consequence of the method of construction, the activeand passive optical and electronic circuit structures of the devices,such as waveguides, p-n junctions, and other structures known in theart, are typically disposed near one major surface of the IO component,herein referred to as the “active” side.

Both laser array 210 and IO waveguide chip 215 are preferably designedand fabricated such that the centroids of the optical modes of each arelocated at a very well known distance from the respective activesurfaces. For laser array 210, this condition may be accomplished by theuse of controlled epitaxial growth of substantially planar layers on aplanar substrate, which are then processed with lithographic techniquesto define the active region. For IO waveguide chip 215, this conditionmay be achieved by using the carefully controlled indiffusion of aspecies into the surface of a planar substrate, e.g., diffusion ofprotons into lithium niobate. Typically, the distance in the verticaldimension between the centroid of the optical mode and the activesurface in laser array 210 will be different than the correspondingdistance in IO waveguide chip 215. To correct for this difference, athin film material of an appropriate thickness may be deposited on theactive surface of one of the components (i.e., the component having thesmaller centroid-to-surface distance) such that the two distances arerendered substantially equal. The thin film material may comprise, forexample, SiO2 deposited by sputtering, or by ion beam assisteddeposition, which technique enables control of the film thickness to anaccuracy of a few nanometers. The thin film material may alternativelybe a metal such as gold. Preferably, the thin film material is patternedinto discrete pads spatially corresponding to some or all of contactareas 232. Well-known photolithographic techniques, such as photoresistliftoff or etching with photoresist protection, may be used to patternthe pads. Alternatively, the pads may be patterned by deposition througha shadow mask. The patterning serves to provide pads with sufficientarea to support the associated component (e.g., IO waveguide chip 215),while minimizing any stress resulting from thermal expansion mismatchbetween the component and the thin film material. Other techniques whichmay be used to compensate for differences in the centroid-to-surfacedistance include interposing, during assembly, preformed metal pads ofappropriate thickness between the component having theshorter-centroid-to-surface distance and the underlying portions ofstandoff structure 222.

Assembly of laser array 210 to submount 205 may be performed accordingto the following process. As described above, standoff structure 222 isprepared on a first major surface 227 of submount 205. If desired, asuitable solder 405 may be deposited into wells 229 defined by ribs 230of standoff structure 222, as illustrated in FIG. 4, to enable bondingof diode laser array chip 210 to submount 205. In another embodiment theadhesive is placed in recesses laterally outside the standoff structure.Laser array 210 and submount 205 are next placed on respective chuckswithin a flip chip bonder. At least one of the chucks of the bonderpreferably provides a compliant or deformable layer as described indetail below. Laser array 210 and the submount 205 are brought intoinitial alignment, which may be preformed, for example, by using acombination of optical alignment and fiducial marks for lateral andin-plane (yaw) angular alignment, and an autocollimator to setparallelism (roll and pitch angular alignment). The flip chip bonder isthen used to bring the top or active surface of laser array 210 intocontact with the reference surface 305 defined by standoffs 230. As itis practically impossible to orient the laser array 210 and the submount205 in perfectly parallel relation, especially when one considers thatthe fabrication process of the laser array 210 generally results in awarped or bowed chip, one portion of the active surface of laser array210 will contact the corresponding area of standoff structure 222 beforeremaining portions of the active surface are brought into contact withstandoff structure 222.

Following the establishment of initial contact between laser array 210and standoff structure 222, force is applied by the flip chip bonder tobring the chucks holding submount 205 and laser array chip 210 stillcloser together, thus pressing laser array 210 onto reference surface305 of standoff structure 222. Force is transmitted to laser array 210through a compliant or deformable layer disposed between the laser arraychip 210 and the metal (or ceramic) chuck which holds the chip 210. Thiscompliant layer can be compressed and distorted, thereby distributingthe applied force over the area of the laser array chip 210 andpermitting it to flex and/or rotate such that it becomes substantiallyuniformly contacted to the reference surface 305 across its entire area.The distributed applied force is sufficient to rotate the laser arraychip 210 to overcome any remaining nonparallelism.

In essence, laser array 210 is pressed onto standoff structure 222,optionally deforming laser array chip 210 or submount 205 or both fromits or their originally provided shape, until the resulting curvature ofthe laser array chip 210 substantially matches the resulting curvatureof reference surface 305. Once such uniform contact is achieved the flipchip bonder may be used to heat the two components to a temperaturesufficient to cause solder 405, which was previously deposited in wells229 between stand-offs 230, to ball up and contact laser array 210. Oncooling, solder 405 solidifies and the force applied by the flip chipbonder may be released. The solder forms a strong bond between laserarray 210 and submount 205 which may be used for both mechanical supportand, if desired, electrical connection between the laser diode drivecircuitry and the laser emitters themselves, and also for providing agood thermal contact for cooling.

It will be noted that the combination of the standoff structure 222 andthe compliant layer disposed between laser array 210 and the chuck ofthe flip chip bonder enables uniform contacting of the laser diodearray, to the reference surface 305 defined by standoff structure 222,across the entire width of laser array 210. Thus, all the emitters ofthe laser diode array 210 are accurately located relative to thereference surface 305. This location is achieved independently of thepresence of curvature or warping in either the submount 205 or the laserarray chip 210, both as originally provided and as finally bonded.

It will also be noted that the position of the emitters relative toreference surface 305 is determined only by the accuracy of thicknesscontrol in the layer deposition and formation processes used toconstruct laser array 210. There is no solder or adhesive locatedbetween the active surface of laser array and the contact portions 232of standoffs 230; rather, the solder or adhesive 405 is located in thewells 229 between the standoffs 230, and so variations in the thicknessor volume of solder 405 deposited on the submount 205 do not affect thealignment of the laser diode array 210.

In addition, the use of standoff structure 222 comprising a set ofstandoffs 230 arranged across the width of laser array 210,advantageously reduces the actual contact area between laser array 210and submount 205 compared to an assembly wherein the entire planarsurface of laser array 210 is contacted with a corresponding planarsurface of the submount 205. This reduced contact area decreases theprobability of a material defect or foreign object (e.g., a dust ordebris particle) being located between the laser array 210 and thesubmount 205 at a point of contact, and therefore decreases theprobability of a resulting misalignment that will adversely affect thecoupling efficiency. As shown in FIG. 2, the aggregate contact areabetween the standoffs 230 and each of the optical array components(i.e., laser array 210 and IO waveguide chip 215) is substantially lessthan the total area by which the submount 205 overlaps each such opticalarray component. The overlap area is defined herein as the intersectionarea of a perpendicular projection of the optical components ontosubmount 205. The aggregate contact area is preferably less than 50percent, and more preferably less than 10 percent, of the correspondingoverlap area in order to minimize misalignment arising from defects orthe presence of foreign particles.

Attachment of IO waveguide chip 215 to the submount 205/laser array 210subassembly (hereinafter “subassembly”) may be performed by a methodclosely similar to the above-described method for attaching laser array210. However, active alignment techniques may be employed forpositioning of IO waveguide chip 215, as is set forth below.

During the attachment procedure, the subassembly may be mounted on afixed or movable stage and provided with electrical connections todriver circuitry to energize the laser emitters. IO waveguide chip 215is preferably mounted with its active or top surface (at or proximal towhich are located the waveguides) facing the standoffs 230 fabricated onthe major surface 225 of the submount 205, with the receiving ends ofthe waveguides facing corresponding emitters of laser array 210. IOwaveguide chip 215 may be mounted in a suitable holder attached to acomputer controlled multi-axis micropositioner capable of submicronpositioning accuracy and repeatability, such as an autoalign system. IOwaveguide chip 215 is initially aligned relative to the assembly usingoptical alignment and fiducial marks to set lateral and longitudinalposition and yaw angle, and an autocollimator to set parallelism tosubmount (roll and pitch angles). IO waveguide chip 215 is then broughtinto close proximity to standoffs 230, with the receiving end of IOwaveguide chip 215 located adjacent to the emitting edge of laser array210. It will be understood that this alignment approach is based on buttcoupling between the laser array 210 and IO waveguide chip 215,requiring close approach of the facets of the two components to preventdiffraction losses between the two. Energizing laser array 210 resultsin the emission of light from the emitters, which is captured by thereceiving waveguides in IO waveguide chip 215. A photodetector (or arrayof detectors) is used to monitor the light transmitted through IOwaveguide chip 215. The position of IO waveguide chip 215 is thenadjusted under computer control to maximize the transmitted light.Importantly, adjustment need be performed only in the lateral,longitudinal and yaw dimensions since the vertical position and thepitch and roll angles are defined by standoff structure 222. Oncealignment is optimized (as indicated by maximization of the lighttransmitted through the waveguide(s) of IO waveguide chip 215), theposition of the chip may be memorized by the computer control system,and the IO waveguide chip 215 withdrawn to allow the dispensing ofadhesive (e.g., an epoxy) into wells 229 located between standoffs 230.Alternatively, solder deposited into wells 229 can be used to providemechanical fixing, if appropriate portions of IO waveguide chip 215 havebeen metallized. After deposition of the epoxy, IO waveguide chip 215 isreturned to its exact previous position using the highly accuraterepeatability of the positioning stages. Final alignment is confirmedand IO waveguide chip 215 is contacted and pressed onto standoffstructure 222 using the micropositioners. If an epoxy is employed toprovide adhesion between IO waveguide chip 215 and submount 205, theepoxy is subsequently cured, e.g., by UV exposure. Note that the mountsupporting IO waveguide chip 215 on the micropositioner is preferablydesigned to be compliant or deformable such that when IO waveguide chip215 is contacted to standoff structure 222, IO waveguide chip 215 isallowed to roll or pivot and flex if necessary to conform to thereference surface defined by standoff structure 222, and/or to removeany warpage or bowing in IO waveguide chip 215. Alternatively, a passiveoptical process may be employed.

By attaching IO waveguide chip 215 in this manner, it can be understoodthat high coupling efficiency between the emitters of laser array 210and the waveguides of IO waveguide chip 215 can be achieved, as thecrucial vertical alignment of corresponding emitting and receivingoptical port is performed passively and with very high accuracy.

Following alignment and attachment of IO waveguide chip 215, the lighttransmitted through IO waveguide chip 215 may be coupled into an arrayof (preferably single mode) optical fibers. The optical fibers may forinstance be assembled in a silicon v-groove array as known in the art toaccurately define the spacing between the fiber cores, and to positionthe fiber cores along a substantially straight line in the lateraldimension. Simultaneous alignment of all of the IO waveguide chip 215outputs into respective optical fibers may be achieved in a singleactive alignment step, where the position of the optical fiber arrayrelative to the output facet of the IO waveguide chip is adjusted tomaximize the light transmitted through the fibers to a photodetector.Once the alignment has been completed the position of the fiber arraymay be secured using, for instance, UV cured epoxy, solder, thermallycured epoxy, laser welding etc. to bond it to submount 205 oralternatively, to the output facet of IO waveguide chip 215.

In general, the alignment tolerances for optical coupling between theoutput waveguides of IO waveguide chip 215 and the fiber array areconsiderably looser (i.e., greater) than those required for acceptablecoupling between the diode laser and the input waveguides of the IOwaveguide chip or between the diode laser and a lensed or chisel-tippedoptical fiber. The looser tolerances are due to the difference in modesizes between those in laser array 210 and the fiber array: in a typicalimplementation of pump source 200, the optical mode in a single modeoptical fiber has a characteristic vertical dimension in the range ofapproximately 6 μm (for a fiber carrying 980 nm light) to 9 μm (for afiber carrying 1480 nm light) fiber, as compared to an optical modehaving a characteristic vertical dimension of approximately 1 μm inlaser array 210. To accommodate the larger mode size of the opticalfiber, the mode size is expanded in the IO waveguide chip, from a smallsize matching the diode laser mode at the input waveguide, to a largersize matching the optical fiber mode, at the output waveguide. A smallmisalignment (e.g., on a sub-micron scale) reduces the overlap betweenoptical modes that are several microns in size by only a small fractionand, accordingly, has only a minor effect on the coupling efficiencybetween a fiber array and an IO waveguide chip with matching mode size.In contrast, the same amount of misalignment would have a major effecton the coupling efficiency between a laser diode array and an IO chip orfiber array due to the much smaller mode sizes involved. It is alsopossible to perform alignment of the fiber array using a standoffdesigned reference surface as used for the diode laser-IO waveguide chipalignment, as described in detail below.

An embodiment of the invention will now be described in terms ofspecific exemplary implementations thereof. It will be appreciated thatthe following implementations are intended to be illustrative ratherthan limiting.

In a first exemplary implementation of diode laser pump source 200,laser array 210 is configured to emit light at a wavelength of around980 nm. Laser array 210 may be fabricated from epitaxially grown layersof AlGaAs or InGaAs as known in the art. As discussed above, laser array210 comprises a plurality of individual emitters arranged in mutuallyparallel fashion. The total number of emitters may vary over an extendedrange (between 2 and several hundreds, inclusive) depending onrequirements of a specific application, but will typically lie in therange between 4 and 64, inclusive. Each emitter preferably lases in asingle transverse mode, known in the art as the (0,0) mode. The positionof the centroid of the laser mode relative to the active surface oflaser array may be determined either from optical measurements of theoutput mode pattern or from computations based on the refractive indicesand thicknesses of the layers grown to form the laser junction andwaveguide structure. This position may readily be determined tofractions of 1 μm.

Laser array 210 may be fabricated by methods known in the art involvingthe photolithographic patterning of precisely grown epitaxial layers todefine the laser structure. Electrical contacts are applied by othertechniques known in the art to ensure ohmic contact to the laserjunction and to the backside of the wafer for the current return path.The laser arrays are then cleaved from the wafer to prepare the emissionfacets and cleaved or diced to size laterally to define the number ofemitters per array. Lithographically defined alignment marks may beprovided to guide the location of the cleaving steps. Suitable coatingsmay be applied to the cleaved or etched facets of the laser array toimprove lifetime and to provide preferential emission of the outputradiation from the output facet. Typically the output facet is coated toprovide a relatively low reflectivity (1-30%) to provide the outputcoupling from the laser diode cavity, while the other, or back facet, istypically coated with a high reflectivity layer (for example, greaterthan 90%). Preferably the individual diode laser emitters are fabricatedat a sufficient lateral spacing to minimize thermal cross-talk betweenemitters and to enable efficient heat extraction from the laser arraywithout requiring an excessive laser diode junction temperature. Themaximum optical output power should be chosen to remain below thethreshold for catastrophic optical damage at the output facets of theemitters. In one implementation, the maximum output power of eachindividual emitter is in the range of 50-500 mW.

The submount 205 may preferably be fabricated from single-crystalsilicon. Alternatively, materials such as lithium niobate and berylliumoxide may be used to provide a closer thermal expansion match to theGaAs material of the laser diode array. The submount material ispreferably chosen to exhibit high thermal conductivity in order toenable efficient extraction of heat generated by the laser emittersduring operation. In the case of a single-crystal silicon submountmaterial, standoff structure 222 may be defined using photolithographicpatterning and wet etching of the silicon wafer surface. The uppersurface of standoff structure 222 comprises the unetched regions of theoriginal surface of the silicon wafer and therefore retains the originalsurface's smoothness and flatness. The height of standoff structure 222,is determined by the etch processing conditions of time, temperature andetch agent (among others) and may be controlled to sub-micron accuracy.In a typical implementation of submount 205, the height of standoffstructure 222 will be around 5 μm.

Alternatively, for both silicon and the alternative materials (lithiumniobate and beryllium oxide) standoff structure 222 may be fabricatedusing the deposition of thin film materials over the surface of theoriginal wafer. For instance, a layer of SiO2 or SiN may be deposited byRF sputtering, PECVD, ion beam assisted deposition, or other process asknown in the art, with very well controlled absolute thickness (±10's ofnm) and excellent uniformity across the wafer (<±0.5%). Standoffstructure 222 may then be formed by photolithographic patterning andetching of the thin film layer(s). For instance, in the case of SiO2, aphotoresist layer may be spun onto the thin film, exposed through a maskcarrying the desired standoff pattern and developed. The patternedresist layer may then be used as a mask to etch the SiO2 layer forinstance using a wet buffered oxide etch or a dry etch, e.g. reactiveion etching in CHF3. The height of standoff structure is determined bythe thickness of the film layers deposited on the surface. By depositinglayers of different materials sequentially with controlled thickness,and then using chemically selective etching it is possible to constructa standoff structure having regions of different heights.

Wells 229 disposed between standoffs 230 may be utilized as reservoirsfor a bonding agent to attach laser array 210 to submount 215.Electrical traces may also be disposed on the submount surface betweenthe standoffs 230 such that electrical contact may be made to theemitters of laser array 210. In some instances it may be desirable forthe electrical traces to enable individual connections for each emitterso that the operation and power output of each emitter may be controlledindependently using suitable external drive circuitry. In others it maybe desirable to gang several or all of the emitters together on commonelectrical connections. Either of these situations can readily beaccommodated by the wafer scale photolithographic processes used toimage and pattern the traces on the submount. For some choices ofsubmount material, such as silicon, it may be necessary to provide anelectrical isolation layer beneath the electrical traces to preventshort circuits and current leakage through the bulk of the submount.

Metal pads may be fabricated in selected wells which correspond tobonding and/or electrical contact areas on laser array 205. These metalpads may be coated with a layer of solder, e.g. and indium based solderdeposited by evaporation or electroplating, or a gold-tin solderdeposited by evaporation. The thickness and area of this solder ischosen such that it does not completely fill (either vertically orlaterally) the well, but so that when it is melted it can ball up due tosurface tension and contact the surface of a component being heldagainst the reference surface defined by the standoff array. The type ofsolder to be used for this attachment is defined by the performancerequirements of the assembled device. At a minimum the solder mustremain solid and maintain a robust physical and/or electrical attachmentthroughout the desired operating and storage temperature range of thedevice, typically about −35° C. to about +85° C.

The fabrication of submount 205 may readily be performed on a waferscale using standard planar lithographic processing techniques as knownin the art. Once the fabrication processing is complete the individualsubmounts may be cut from the wafer using a dicing saw. As shown inFIGS. 2-3, the submount 205 may be sized such that it does not extendbeyond the output end of IO waveguide chip 215, in order to facilitatethe approach of the output fiber array to the output facet of the IOwaveguide chip 215 as described below.

Attachment of laser array 210 to submount 205 may be performed using amodified flip chip bonder. The prepared submount with standoffs,electrical connections, solder preforms and lithographically patternedoptical alignment or fiducial marks is loaded onto the substrate chuckof the bonder and held in place typically with vacuum pull from thebackside. The prepared laser array, with electrical and bonding contactpads and facet coatings, is loaded into a specially modified samplechuck on the flip chip bonder.

The chuck is modified such that interposed between the laser array chipand the metal or ceramic chuck is a layer of compliant or rubber likematerial. This material, for example, a polyimide, a silicone rubber, ora fluoroelastomer such as Viton® (manufactured by DuPont DowElastomers), is deposited on the chuck and subsequently processed toyield a flat surface, e.g., by lapping and polishing on a flat platewith progressively finer polishing compounds. Vacuum holes are formed inthe compliant layer to match those present in the standard chuck toprovide a holding force to secure the array chip.

After loading laser array 210 and submount 205, an autocollimator isused to align the two components approximately parallel in the pitch androll axes, and lateral, longitudinal and yaw angle adjustments are madeto position laser array 210 correctly with respect to the alignmentmarks on submount 205, such that the electrical and/or attachmentcontact pads on laser array 210 will align with respective features onsubmount.

The two chucks are then brought close together such that laser array 210contacts standoff structure 222. In general, the angular alignment oflaser array 210 and submount 205 is imperfect, and laser array 210 oftenis warped or bowed due to stresses introduced during the fabricationprocessing, such that one point or line on laser array 210 contacts thestandoff structure before remaining portions. At this point, thecompliant layer between laser array 210 and the sample chuck iscompressed or distorted, accommodating angular misalignment, andallowing laser array 210 to rotate and/or flex such that it comes intosubstantially uniform contact with standoff structure 222. Although thecompliant layer allows the rotation/flexing of laser array 210 tocontact the standoff structure, it preferably resists lateraldisplacements or motion that would otherwise cause lateral orlongitudinal misalignment of laser array 210 with respect to submount205. After contact is achieved the vacuum hold on laser array 210 may bereleased if desired.

Subsequent to contact (or if preferred before contact is complete orduring the contacting process) the temperature of the components israised sufficiently high to melt the solder which was previouslydeposited and patterned in wells. On melting, the solder preferablyballs up until it contacts and wets the contact pads on laser array 210.Fluxes and other processes such as forming gas or formic acid vapor maybe used as known in the art to improve the solder flow, wetting andadhesion properties. The subassembly is then cooled to harden thesolder. If it has not already been released the vacuum hold on thesample may be removed during or after the cool down cycle.

Once cool, the subassembly may be removed from the flip chip bonder forfurther processing. Electrical connections to the active side of laserarray 210 are preferably made by the solder joints described above.Electrical connection to the backside of laser array 210 to provide thecurrent return path may be made by placing wire bonds from metallizedpads on the backside of the semiconductor wafer to bonding pads locatedon the submount.

In the presently-described implementation, IO waveguide chip comprises alithium niobate (LN) crystal with waveguides fabricated therein using anannealed proton exchange (APE) method. Preferably the LN crystal is X-or Y-cut such that the waveguides support a TE polarization mode thatmatches the output polarization of typical laser emitters.Alternatively, a TM polarized diode laser may be used, coupled to an APEwaveguide in Z-cut lithium niobate, which also supports a TM polarizedmode. In alternate implementations, IO waveguide chip 215 may be formedusing other suitable optical materials and methods of waveguidefabrication, including without limitation, ion exchanged or indiffusedglass, multi-layer polymer stack, or silica-on-silicon structures.

The receiving or input end of IO waveguide chip 215 is adapted with anarray of waveguides that having lateral spacing matched to that of thelaser array 210 emitters. In addition, the waveguide dimensions andprocess parameters (such as anneal time/temperature) are chosen so as tocreate a very tightly confined optical mode with mode dimensions similarto that of the emitters (typically around 1 μm in the vertical dimensionand 3 μm in the lateral dimension). The tightly confined waveguide modein IO waveguide chip 215 assures a high overlap integral between thelaser array 205 output mode and the IO waveguide chip 215 input modewhen the facets of the two chips are brought into close proximity in thebutt coupling procedure described below. In practice, the actual modesize in the input waveguides of IO waveguide chip 215 may be somewhatlarger than that of the laser array 210 emitters due to processconstraints.

A primary function of IO waveguide chip 215 is to act as a mode sizeconverter between laser array 210 and the single mode optical fibersemployed at the output of pump source 200. Tapered waveguide sectionsare used for this purpose, according to known art, and it is in generalpossible to create almost any desired mode size by appropriate choice ofphysical mask dimensions, proton exchange time and temperature, andannealing time and temperature.

The waveguide taper structure may be constructed in a single processingstep, with a single set of exchange and anneal parameters and onelithographically defined mask. Alternatively the taper structure may beconstructed in several sequential steps using multiple exchanges,anneals, and masking steps. The single step construction method may notenable full optimization of the waveguide mode size at the extreme inputand output regions of IO waveguide chip 215, but it offers a far lesscomplicated process than the sequential method, which requires severalcritical alignments. An example of single-step constructed waveguidetaper structure formed in IO waveguide chip 215 is a 4 μm wide maskdimension at the input/receiving end which tapers down to a 2 μm widemask dimension at the output end. The wider channel at the input createsa well-confined small mode, while the reduced lateral dimension of thewaveguide mask at the output end significantly reduces the total numberof protons added to the substrate in the same proton exchange andanneal, and thus provides a waveguide core at the output that has asmaller refractive index. Consequently a much more weakly confinedwaveguide is produced at the output end of the taper, having a mode thatis well matched to that of a single mode optical fiber. If desired,further optimization of the mode sizes in the vertical dimension tomaximize mode overlap and coupling efficiency at the input and outputmay be achieved by the addition of thin film layer sections to thesurface of IO waveguide chip 215. For example, a layer of high indexNb2O5 may be provided at the receiving end to confine the waveguide modestrongly near the surface of the chip; or a layer of SiO2 (or SiONx), atthe output end, to effectively bury the waveguide and force the modedown, away from the surface of the LN crystal, making it moresymmetrical. Preferably, the waveguide structure is also designed to beadiabatic, supporting only a single lowest order (0,0) mode throughoutits length, so that excess loss due to mode conversion is minimized oreliminated.

It is known in the art that waveguides with very tight confinement andsmall mode sizes often exhibit high propagation losses. It is thereforepreferable for the tightly confining receiving end section of thewaveguide to be as short as possible, and for the waveguide dimensionaltaper to start very close to the receiving facet. In principle it ispossible to reduce the taper length to under 500 μm whilst maintaininglow-loss tapering of the waveguide.

After fabrication of the waveguide structures, it is desirable to applywell controlled thin film layers to the surface of the wafercorresponding to the top or active surface of IO waveguide chip 215,such that the centroid of the intensity profile of the optical mode ofIO waveguide chip 215 is located at the same distance below theeffective wafer surface (that is, the surface of the thin film layerdeposited on the wafer) as the centroid of the mode of the emitters oflaser array 210. Alternatively, the thin film may be applied to merelyprovide for optimum optical coupling to be achieved irrespective of theoptical mode centroids. The required thickness of material may rangefrom 0.1 μm to several microns, preferably controlled to better than 0.1μm in absolute thickness and in uniformity. A suitable material for thislayer is SiO2, deposited for instance using ion beam assisted depositionwhich enables exceptionally high uniformity and tight depositionthickness control up to tens of nanometers. The exact layer thicknessrequired to match the centroids of the modes of laser array 210 and IOwaveguide chip 215 may be determined from experimental measurement orfrom modeling. The height matching layer described above may also servethe purpose of preventing dirt and debris from coming into directcontact with the surfaces of the optical waveguides.

In addition to carrying the mode converting tapered waveguide structure,IO waveguide chip 215 may also be provided with alignment and fiducialmarks referenced to the waveguide positions, for instance fabricated byplanar photolithography and etching of a thin metal layer deposited onthe crystal surface. These marks may be used for initial alignmentduring the attachment process described below. They may also be used todefine dicing and polishing markers so that the end faces of IOwaveguide chip 215 may be cut and polished to enable end-face launchingof laser light into the waveguide facets. Such cutting and polishing isa routine process that is well understood in the art, with a requirementthat the angles of the polished face with respect to both thelongitudinal waveguide axes and the active surface be accuratelycontrolled.

After dicing and polishing, it is preferable that anti-reflection (AR)coatings be provided on the input and output end facets of IO waveguidechip 215. LN has a high refractive index (approximately 2.2) andconsequently light incident on a surface of an uncoated LN crystal willexperience a significant amount of Fresnel reflectance. To maximize theamount of light coupled into and through the waveguides of IO waveguidechip 215, it is desirable to minimize the amount of reflection. Toachieve this objective, an anti-reflective (AR) coating comprising amulti-layer stack of alternating SiO2 and TiO2 layers may be depositedon the appropriate surfaces of IO waveguide chip 215. As is known in theart, the number and thicknesses of the layers in the stack are adjustedaccording to the refractive index of the substrate being coated.Residual reflections may be thereby reduced to values as low as0.01-0.1%, depending on the complexity of the coating structure and theoptical bandwidth over which it must provide the anti-reflectionfunction. In addition to maximizing the amount of light coupled into andout of the waveguides in IO waveguide chip 215, the AR coating alsominimizes unwanted back reflections into the emitters of laser array210, which may otherwise distort the lasing properties and causeunwanted fluctuations in emission wavelength and/or output power.

The prepared IO waveguide chip 215 may be aligned and attached to thesubmount 205/laser array 210 subassembly as follows. The submount/laserarray subassembly is mounted on a holder and connected to an electricaldrive circuit to enable the laser emitters to be energized. Thiselectrical connection may either be temporary with zero-insertion forceconnectors or the like, or may involve mounting and wirebonding of thesubmount/laser assembly to some other carrier structure.

IO waveguide chip 215 is mounted in a specially designed holder and heldin place using vacuum force or other suitable technique, with the activesurface facing submount 205. Preferably, the chip holder is attached toa six-axis motion control system, such as an autoalign system, providingcomputer controlled motion and positioning with sub-micron accuracy(≦0.1 μm) in the three orthogonal displacement axes and the threeangular axes. Initial alignment of IO waveguide chip 215 to standoffstructure 222 is achieved using a combination of an autocollimator toadjust pitch and roll angles, and optical alignment marks to providelongitudinal, lateral and yaw adjustment.

IO waveguide chip 215 is then brought into close proximity to standoffstructure 222, with the receiving facet of IO waveguide chip 215 broughtinto close proximity to the output or emitting facet of laser array 210so as to enable butt coupling of the emitted laser light. Laser array210 is energized to emit light by the application of an electricalsignal from the above-mentioned drive circuit. At the output end of IOwaveguide chip 215 may be located one or more lenses disposed to directand focus light emitted from one or more waveguides in IO waveguide chip215 to one or more photodetectors configured to monitor the poweremitted from the waveguides of IO waveguide chip 215. The position of IOwaveguide chip 215 is adjusted using computerized techniques in order tomaximize the optical power transmitted through the waveguides. Duringfinal coupling optimization IO waveguide chip 215 is preferablycontacted to standoff structure 222, allowing final setting of thelateral, longitudinal and yaw dimensions while constraining vertical,pitch and roll motions.

Note that the holder supporting IO waveguide chip 215 is preferablydesigned to allow a degree of flex or distortion in the position of thechip in response to pressure applied thereto. In this manner the holdermay be designed with roll centers and pivot points such that, as IOwaveguide chip 215 first contacts standoff structure 222, it is able toroll or flex sufficiently that it contacts standoff structure 222substantially uniformly across its entire width, in a similar way as thecompliant layer disposed between laser array 210 and the flip chipbonder chuck enables uniform contact between laser array 210 andstandoff structure 222 even in the presence of initial misalignments andbowing or warping of the array itself. This holder may incorporatestrain gauges and flexure structures in the mounting arms such that thestrain in the holder may be monitored to determine when contact betweenIO waveguide chip 215 and standoff structure 222 has occurred.

Once optical coupling has been optimized, IO waveguide chip 215 must besecurely attached to the submount. This function may be performed usinga UV curing adhesive, such as an epoxy, in the following manner. First,the position of IO waveguide chip 215 is stored in memory by thecomputer control system, and IO waveguide chip 215 is then withdrawnfrom contact with submount 205. An epoxy dispense system may then bemoved into position over submount 205 and controlled volumes of epoxydispensed into specified wells 229. IO waveguide chip 215 is thenrepositioned at the stored location, and (if necessary) fine adjustmentsof the position and orientation of waveguide chip 215 are effected tore-optimize optical coupling between laser array 210 and IO waveguidechip 215 (noting that the precise positional repeatability of commercialcomputer control systems, which is typically around 0.1 μm, generallyobviates the need to perform such re-optimization). The epoxy may thenbe cured using exposure to UV or short wavelength visible light whichmay be transmitted through IO waveguide chip 215. Those skilled in theart will recognize that it is important to choose an epoxy that exhibitsgood adhesion to the contact materials, that is the submount, the IOwaveguide chip 215 surface and/or any thin film materials deposited onIO waveguide chip 215 for height adjustment and surface protection.Alternatively, a UV-curing acrylate adhesive may be employed. As anotheralternative procedure, IO waveguide chip 215 may be fastened in placeusing a solvent-free polymeric adhesive, such as a commerciallyavailable solvent-free thermal-curing or UV-curing epoxy. This approachmay leave less time to reoptimize alignment (due to generally higherpre-cure viscosity because of the absence of a solvent) but mayadvantageously reduce or eliminate subsequent outgassing of the adhesive(which may shorten device lifetime if not properly mitigated).

Alternatively, IO waveguide chip 215 may be fastened in place usingsolder. In one implementation of this approach, metal pads may bedefined photolithographically on the active surface of IO waveguide chip21 (on the surface of any thin film coatings deposited over thewaveguides, or on the surface of the LN material itself if no suchcoatings have been applied). It is noted that the metal pads should liebetween the waveguide features so as not to introduce excessiveabsorption losses. Solder may be deposited and patterned onto thesecontact pads as known in the art to create solder preforms distributedin some pattern on IO waveguide chip 215 matched to respective bondingpads on submount 205. Preferably, the thickness of the solder asdeposited is not sufficient to touch the submount bonding pad when IOwaveguide chip 215 is contacted to standoff structure 222, but issufficient to cause the solder, upon melting, to contact the submountbonding pad thereby producing a robust mechanical joint. In the case ofthis solder attach approach, after initial alignment optimization theassembly may be heated to the melting point of the solder, whichpreferably is lower than the melting points of any solders used in theattachment or wirebonding of the submount/laser assembly. Once thesolder has balled up and made contact between the bonding pads on therespective components the temperature is reduced to re-solidify thesolder and form the mechanical bond. Preferably the laser is notenergized during thermal cycling during solder bonding, as hightemperature operation can lead to premature and sometimes instantfailure. Note that techniques known in the art to improve solderbonding, such as the use of fluxes, forming gas and formic acid vapormay be applied to improve this process.

It is noted that a laser welding-based technique may alternatively beutilized to attach IO waveguide chip 215 to submount 205. Since the LNmaterial of waveguide chip 215 is substantially transparent to opticalenergy having wavelengths in the 0.35 μm-4 μm region, a laser beamhaving a wavelength within this range is able to traverse the materialwithout significant absorption such that it impinges upon and isabsorbed by a solder or adhesive target. In this manner an IR laserwelding station could be used to attach IO waveguide chip 215 tosubmount 205. Because this technique does not require heating of largeportions of IO waveguide chip 215 and/or submount 205, its use may beadvantageous in connection with component materials having relativelylow melting or deformation temperatures.

The assembly comprising submount 205, laser array 210 and IO waveguidechip 215 may now be prepared for the alignment and attach of the fiberarray, which functions to carry the laser light to the amplificationregion. It is noted that the task of coupling the fiber array to themode matched output waveguides of IO waveguide chip 215 requiresconsiderably less precision, relative to the task of coupling laserarray 210 and IO waveguide chip 215, due to the significantly greatermode sizes involved. The mode diameters of the output waveguides and thesingle mode fibers are typically between 3-7 μm. Preferably, the arrayof fibers is prepared using silicon V-groove technology as known in theart, such that (1) the centers of the optical fiber cores are accuratelypositioned at the desired pitch to match the output waveguides from IOwaveguide chip 215, and; (2) the fiber core centers are aligned along asubstantially straight lateral line to match the output waveguide facetsof IO waveguide chip 215. It is also preferable that the end faces ofthe optical fibers be either anti-reflection coated or cleaved/polishedat an angle such that the Fresnel reflection from the input fiber corefacet is eliminated, or at least not coupled back into the waveguide inIO waveguide chip 215, from where it may be coupled back into theemitters of diode laser array 210 resulting in power and/or wavelengthinstability in the laser output.

Active optical alignment and attachment may be performed using any ofthe methods currently practiced in the art, such as laser welding or UVadhesive bonding. In the case of UV adhesive, the assembly is mounted ona holder and connected to drive circuitry to activate some or all of thediode laser emitters. The light emitted from the diode laser andtransmitted through IO waveguide chip 215 exits the output waveguidefacets of IO waveguide chip 215.

The silicon V-groove mounted fiber array is mounted on a multi-axismicropositioner, preferably with computer control. The fiber array isbrought into proximity with the output facet of IO waveguide chip 215and at least coarsely aligned with respect to lithographically definedfiducial or alignment marks disposed on IO waveguide chip 215. Lightcoupled into the fibers of the array is monitored by one or morephotodiodes or power meters disposed at the output of the fiber array.Preferably the signal from the monitoring photodiodes is used by thecomputer control system to adjust the position of the fiber array untilmaximum optical coupling, and therefore transmitted signal, is achieved.

Epoxy may then be dispensed either around the silicon V-groove fiberholder and/or between the fibers and the end facet of IO waveguide chipas desired. Bonding of the silicon V-groove fiber holder may be achievedby exposing the epoxy to UV light. Preferably the epoxy is chosen toexhibit little shrinkage on cure such that the position of the fiberarray is not significantly altered by the curing process. In someinstances it may be preferable to have an epoxy free optical path, thatis to ensure that no epoxy is located between the output waveguidefacets of IO waveguide chip 215 and the input facets of the opticalfibers in the array. In such instances the epoxy should be dispensedonly in side areas free of waveguide facets. In this embodiment wherethe fiber array is bonded directly to the output facet of IO waveguidechip 215, it may be preferable to bond a spacer piece of LN material tothe top surface of IO waveguide chip 215 at the output end prior to thecut and polish processes described above. In this way, the end faces ofIO waveguide chip 215 and the spacer LN piece are cut and polishedsimultaneously, effectively forming a single surface which spans bothabove and below the waveguide output facet. In this case, it ispreferable that submount 205 does not extend to the output end of the LNchip as the spacer LN piece would interfere with the flip chip bondingprocess. In other embodiments described below it may be important thatsubmount 205 extends at least up to and sometimes beyond the outputfacet of the LN chip. The spacer LN piece described above serves toprovide a larger mounting surface for the attachment of the fiber array,which in turn leads to a more robust and reliable mechanical joint andimproved stability and lifetime of the optical alignment between the twostructures.

Once the attachment of the fiber array is performed, the diode laserpump module assembly is completed by the provision of an externalprotective package such as that shown in FIG. 21, and suitableelectrical and control connections are provided to enable the desiredlaser operation as known in the art. Preferably the diode laser pumpsource may be enclosed in a hermetically sealed butterfly type packageas known in the art.

It should be noted that elements of the external package may beincorporated into the above described process flow at any point withoutinterfering with the performance of this invention. For instance, it maybe preferable to place and mount the submount/laser diode array/IOwaveguide chip assembly inside the open butterfly package beforeperforming alignment and attach of the fiber array, or vice versa. Inaddition, it will be apparent that, for example, the output ends of thefibers may be attached to the amplifier(s) by connectorizing or fusionsplicing.

FIG. 5 depicts an amplifier pump source 500 in accordance with a secondembodiment of the present invention, wherein increased functionality isprovided to pump source 500 by the inclusion of distributed Braggreflectors (DBRs) 505 in the form of Bragg gratings. For the purpose ofclarity, the submount has been omitted from FIG. 5, and laser array 210and optical fiber array 220 are depicted in phantom. DBRs 505 arepreferably superimposed on the optical waveguide structures 507 of IOwaveguide chip 215 and are configured to provide narrow-band,wavelength-selective retroreflection. The light reflected by each DBR505 is fed back into a corresponding emitter of laser array 210 for thepurpose of stabilizing the wavelength of emitted light, in accordancewith processes well established in the prior art. The wavelength oflight reflected by each DBR 505 is determined by its grating period. Itshould be noted that the grating period may be varied among the severalDBRs 505 fabricated on IO waveguide chip 215, thereby enabling differentemitters of laser array 210 to be locked to different wavelengths. Withthe appropriate choice of DBR bandwidth (determined by the length of thegrating), and reflectivity (determined by a combination of the gratingorder, effective depth, and length), the light output by each emittercan be locked to within the bandwidth of the associated DBR 505 in amanner largely independent of the operating conditions of laser array210, specifically drive power and ambient temperature. Typically areflection back into the emitter of a few percent of the total emitteroutput power is sufficient to lock the emitter output to the wavelengthband of DBR 505. The output facet of the laser array 210 may have anappropriate low reflectivity coating, again in the order of a fewpercent, while the input facet of IO waveguide chip 215 shouldpreferably have a very low reflectivity anti-reflection coating 510 suchthat the broadband (in wavelength terms) reflectivity from the inputfacet does not destabilize the emitters of laser array 210. The outputfacet of IO waveguide chip 215 should also preferably have ananti-reflection coating 515 to suppress reflection.

DBRs 505 may be fabricated using planar lithographic processing methodsknown in the art. For example, a fine pitch grating suitable forretroreflection of 980 nm light in a lithium niobate waveguide requiresa period of around 230 nm for a first order structure. This period maybe created in a thin photoresist layer (˜0.1 μm) using two-laser-beamholographic interference lithography or using a contact printingapproach with a phase mask illuminated either with a UV laser or amercury lamp. Development of the exposed resist leaves a grating patternwhich may be transferred directly into the lithium niobate, or into somesurface layer on the lithium niobate, using an approach such as ion beammilling. Alternatively reactive ion etching or laser ablation could beused for pattern transfer. The depth of the grating and thus thereflectivity per grating line is defined largely by the depth, which issimply controlled by the etch time. Different periods may be fabricatedusing a sequential exposure and etch process with shadow mask materialsuch as chrome covering the areas which do not require a grating of aparticular period. Alternatively, using the phase mask exposure process,a number of different periods can be defined simultaneously using acustom designed and fabricated phase mask containing the relevantpatterns for transfer. This grating fabrication processing for IO chipswould typically be performed after waveguide fabrication on a wafer butbefore surface protection coating and chip dice (separation into chips)and end face polish. The exact design of DBRs 505 depends heavily on theprecise waveguide structure and overlayer/protection materials used andmust be adjusted accordingly based on existing modeling capabilitiescombined with empirical measurements of DBR 505 performance afterfabrication.

In FIG. 5, different locations of DBRs 505 are shown on differentwaveguides 520, 525. The placements of DBRs 505 illustrates that thedesign of the DBR and the design of the waveguide are linked. On nearwaveguide 520, DBR 505 is shown disposed on the highly confining inputwaveguide section 530 that supports a very small mode to provide goodcoupling efficiency to the emitter output mode. In this case the gratingneeds only to be very shallow in order to achieve a desired lowreflectivity as a shallow structure interacts strongly with the small,tightly confined optical mode. However, for some waveguide fabricationprocesses, the tightly confined input waveguide section may exhibitrelatively high propagation losses, making it preferable that the inputsection be as short as possible. In this case it is preferable to placeDBR 505 as shown on the far waveguide 525, superimposed on a lower loss,more loosely confining waveguide section 535. Preferably waveguidesection 535 is intermediate in mode size and confinement between thetight input section and the loose output section which requires agrating of greater depth in order to interact efficiently with therelatively large and weakly guided optical mode which is dimensioned tomatch efficiently to a single mode optical fiber.

Alternatively DBR 505 may be the conventional type known in the artformed by lithographically patterned UV exposure of the core andcladding of an optical fiber, and in this case may be located in each ofthe fibers of output fiber array. In other embodiments, the grating maybe superimposed over the taper to increase the bandwidth of the grating,which may be desirable to optimize laser stabilization.

Another embodiment of the present invention, shown in FIG. 6, addsfurther functionality to laser array amplifier pump source 600 byincreasing the output power coupled into a single fiber 620 by means ofwavelength multiplexing the output of several laser emitters from thelaser array 610. Increasing the output power in fiber 620 increases theavailable performance of an amplifier pumped by laser array amplifierpump source 600, while still allowing the individual emitters of laserarray 610 to be operated at output powers well below the threshold forfailure due to catastrophic facet damage or other effects. Using a setof narrow-band DBRs 605, a number of diode laser emitters are stabilizedto different wavelengths, separated by a known wavelength interval anddefined by the periods of the DBR 605 gratings. Preferably all theemitted wavelengths lie within the absorption spectrum of the amplifiermaterial of application, such that they may usefully contribute to theamplification of signal light. For application in EDFAs the emittedwavelengths are preferably within about 15 nm of the peak of theabsorption spectrum located near 975 nm. The different wavelengthoutputs from the laser emitters are then combined usingfused-fiber-optic directional couplers 625 which provide a wavelengthselective coupling function and enable the power in several wavelengthchannels to be combined to a single fiber 620. The design of fused fiberdirectional couplers must be matched to the emitter wavelengths andtheir spacing. Such fused fiber WDM couplers are commercially availablewith wavelengths separations down to 2 nm to combine 4 or more channelsto a single fiber.

FIG. 7 illustrates an embodiment of the invention similar to the FIG. 6embodiment but distinguished therefrom by its use of integrated opticwaveguide directional couplers 710 (in place of the fused fiberdirectional couplers of the FIG. 6 embodiment) to perform the wavelengthmultiplexing operation on the IO waveguide chip 715. Directionalcouplers 710 may be fabricated by defining two optical channelwaveguides in close proximity to create a coupling region. In LN,waveguide directional couplers 710 may be designed using existingmodeling capabilities such as beam propagation modeling (BPM) andknowledge of the properties of annealed proton exchange waveguides. Thelength of the coupling region of directional couplers 710 is known to beinversely proportional to the wavelength separation of the two channelsto be combined. In FIG. 7 the optical waveguides are shown to have twotaper regions: a first taper region 725 situated upstream in the opticalpath relative to directional couplers 710, and a second taper region 735situated downstream in the optical path relative to the directionalcouplers 710. This arrangement enables independent optimization of thewaveguides in the three main sections of the chip input, directionalcouplers, and output. In this way the waveguide dimensions andconfinement can be optimized to achieve high input coupling efficiencyfrom laser array 705, minimum directional coupler length with low losswithin the IO waveguide chip 715, and high output coupling efficiency tosingle mode optical fiber 720. The number of channels which can bepractically multiplexed together in this way is limited by the length ofthe directional couplers. For example, a simple coupler designed tomultiplex two wavelengths around 980 nm separated by 5 nm may have alength of ˜4 mm, 8 mm or 19 mm, for example.

FIGS. 20 a-d depict still further embodiments of the invention, whereinwavelength selective feedback to stabilize the individual emitters ofthe laser diode array is provided after, and by, one or more wavelengthmultiplexing directional couplers that combine the outputs of severalemitters of the array, as described in detail below.

The use of narrow-band DBRs for stabilizing diode lasers was describedabove with reference to FIG. 5. Use of fused fiber couplers and,alternatively, IO waveguide directional couplers, for combining theoutput of two or more emitters operating at different wavelengths, alsocalled wavelength multiplexing, was described in connection with FIGS. 6and 7. In those embodiments, the DBRs are positioned upstream in theoptical path from the wavelength multiplexing directional couplers asshown in FIG. 20 a. Referring to FIG. 20 a, two emitters of a laserdiode array chip 2210 are butt-coupled to waveguides in an IO waveguidechip 2215. Two waveguides 2231 and 2232 receive light from the emitters,and are adapted with DBRs 2201 and 2202, followed by a waveguidedirectional coupler 2240. DBRs 2201 and 2202 stabilize the two emittersat two different wavelengths λ1 and λ2. The outputs at the twowavelengths are combined (multiplexed) in directional coupler 2240 andpropagate in a single waveguide 2231 to the output facet of the IOwaveguide chip, where the combined light beam containing energy at twodifferent wavelengths couples into an output optical fiber 2220.

In the embodiments depicted by FIGS. 20 b-c, the stabilizing DBRs arepositioned after the directional couplers, in series, one after another.In a first alternative shown in FIG. 20 b, two DBRs 2201 and 2202 aredisposed in series on waveguide 2231, after waveguide directionalcoupler 2240, on IO chip 2215. According to known art, the waveguidedirectional coupler 2240 transmits, in the reverse direction, light ofwavelength λ1 preferentially in one input waveguide arm, such as 2231,and light of wavelength λ2 preferentially in the other input waveguidearm, such as 2232. It should be noted that not all waveguide directionalcouplers have this property, and therefore this embodiment requires useof appropriate design for waveguide directional couplers to ensure theirfunctioning in the desired manner. In a second alternative, shown inFIG. 20 c, two stabilizing DBRs 2251 and 2252 constructed in the form offiber Bragg gratings (FBGs), as known in the art, are positioned inseries on output fiber 2220. In a third alternative (not shown) onestabilizing DBR is positioned on waveguide 2231 after directionalcoupler 2240 on IO waveguide chip 2215, and the other stabilizing DBR ispositioned on output fiber 2220 coupled to waveguide 2231.20 d The DBRsin the alternative positions, two of which are shown in FIGS. 20 b-c,are effectively in series in the multiplexed light output path andfunction in substantially the same manner described in connection withFIG. 20 b, whether they are on the IO chip or on the output fiber.Grating fabrication processes in optical fiber may be a more maturetechnology than grating fabrication processes in IO materials, and thusmay provide an economic advantage in some applications. Positioning theDBRs on optical fiber rather than on the IO chip allows wavelengthselection of the emitters of a laser diode array independently ofwavelength multiplexer design, and this may also provide an advantage insome cases.

In a fourth alternative form of this embodiment shown in FIG. 20 d,feedback for stabilizing the laser diode array emitters in a wavelengthmultiplexed configuration is provided after the waveguide directionalcouplers, in a position substantially similar to those shown in FIGS. 20b-c for narrow-band DBRs, by a weak spectrally-wide reflector.

Preferably, such a reflector is a wideband DBR 2203 disposed on theoptical waveguide 2231, between the directional coupler 2240 and theoutput facet 2217 of the IO waveguide chip 2215, and reflectionsoriginating from further downstream in the optical path are suppressedby appropriate techniques known in the art, such as by adapting the endof the optical fiber 2220 with a lens or chisel-tip shape and providinganti-reflective (AR) coating thereon. Wavelength selectivity tostabilize the emitters to particular wavelengths is provided by thewaveguide directional coupler 2240 acting together with the widebandreflector. Maximum feedback occurs at the wavelengths of preferentialreverse transmission in the waveguide directional coupler, which isappropriately designed with sufficient wavelength selectivity, accordingto known art. Preferential reverse transmission to one input waveguide2231 of the directional coupler occurs in narrow bands peaked at a firstset of wavelengths λ1, λ3, . . . , and to the other input waveguide2232, at a second set of wavelengths λ2, λ4, . . . , which alternatewith the first set of wavelengths such that λ2 is intermediate λ1 andλ3, λ3 is intermediate λ2 and λ4, and so on. The optical reflection bandof DBR 2203 is adapted to lie within the open-loop optical gain band ofthe laser diode array emitters, and to be sufficiently wide to reflecttwo adjacent preferential reverse transmission wavelengths (e.g., λ1 andλ2), one for each of the two input waveguides of the directional coupler2240, but to substantially transmit and not reflect the otherwavelengths of both sets. Therefore the emitters of the laser diodearray 2210 that are coupled to these waveguides are stabilized at thewavelengths λ1 and λ2, respectively. Alternatively, the wideband DBR2203 may be omitted, and wideband reflection may be provided by a thinfilm stack optical reflector, with its reflection band and operationsubstantially similar to that described above for DBR 2203, disposed onthe output facet 2217 of IO waveguide chip 2215 (in place of an ARcoating). Further alternatively, the wideband DBR or thin film stackreflector may be omitted and wide band reflection may be provided by anuncoated or suitably coated output facet of the IO waveguide chip or theinput facet of a cleaved output fiber, and wide-band selectivityprovided by adapting the laser diode emitters of array 2210 to have anopen-loop optical gain spectrum that is appropriately wide to containonly the wavelengths λ1 and λ2, thereby selecting only these directionalcoupler retroreflection wavelengths for lasing, one for each emitter.This structure and method to employ the narrow-band wavelengthselectivity of optical waveguide directional couplers to stabilize thelaser emitters is not known in prior art, and provides the considerableadvantage of removing the need to have narrow-band DBRs and to matchtheir spectral transmission characteristics to the wavelengthmultiplexing directional couplers.

FIG. 8 shows a still further embodiment of the present invention whichprovides for increasing the output power of the laser array amplifierpump source that is coupled into a single fiber output. As depicted,fiber optic polarization-maintaining (PM) combiners or multiplexers 825are used to combine the outputs from pairs of adjacent waveguides 802and 804 of the IO waveguide chip 815 into single output fibers 835. Herethe output of waveguides 802 and 804 on IO chip 815 are coupled intopolarization preserving optical fibers, the output from waveguide 802coupling into optical fiber 842 and the output from waveguide 804coupling into fiber 844. Alternate fibers have been twisted by 90° sothat the outputs from adjacent waveguides couple into orthogonalpolarizations in alternate fibers, as indicated symbolically byalternating butterfly shading at the ends of the fibers. In general, allof the outputs from a laser array 810 and IO waveguide chip 815 arepolarized in the same direction, parallel to the plane of the diodejunction. Owing to the twist as indicated by circular arrows in FIG. 8,the polarization of the light becomes physically rotated 90° along withthe fiber in alternate fibers, such as fiber 844, before joiningpolarization multiplexer 825. The physical 90° rotation or twist ofalternate fibers may be readily accomplished by alignment when thepolarization preserving fibers are fabricated into an array using asilicon V-groove holder. Thus the outputs of two adjacent waveguides maybe combined onto the same output fiber but in orthogonal polarizationsto effectively increase, by almost a factor of two, the output power inthe single fiber, with the penalty of only a small loss associated withthe polarization multiplexer itself. Polarization multiplexers capableof combining orthogonally polarized light around 980nm are known in theart and are commercial available.

It is noted that the application of the polarization multiplexer may beindependent of, or combined with, the wavelength multiplexing of theFIG. 6 embodiment described above in order to further increase the powercoupled into an output fiber. It is further noted that the functions ofpolarization rotation and combination/multiplexing can be performed onthe integrated optics chip itself. In LN, it is known to be possible tofabricate frequency selective polarization rotators or TE-TM converters,as well as broadband polarization combiners. From these basic buildingblocks it is possible to construct a multiple stage polarizationcombiner. In such a device it is necessary to fabricate a waveguidestructure that supports both TE and TM polarization modes, whereas thecommonly employed annealed proton exchange(APE) process supports onlyextraordinarily polarized modes (TE in X and Y-cut LN, TM in Z-cut LN).Other waveguide fabrication techniques in LN include metal indiffusion,which generally creates a polarization insensitive waveguide (althoughin some instances, such as titanium indiffusion, it often createswaveguides that are susceptible to photorefractive damage and have lowpower handling capability). A suitable fabrication process to achievethe integrated polarization multiplexer may be zinc indiffusion, whichhas been shown to produce low loss, polarization-insensitive andphotorefractively robust optical waveguides. A transition region fromthe input, tightly confined annealed proton exchange waveguide isrequired to match the input light to the relatively larger modedimensions of the waveguide formed by zinc-indiffusion, which thenmatches well to the single mode optical fiber output. Alternatively,waveguides formed by titanium indiffusion may be employed, but may besubject to the operational problems alluded to above.

It is further noted that multiplexed laser array pump sources arepreferably designed such that the specified maximum optical output powerin each output fiber may be achieved at less than the maximum possibledrive current applied to each laser emitter. Thus, should one or moreindividual laser emitters fail or suffer from reduced power output inany of the emitter-multiplexed embodiments described above, theremaining fully operational emitter(s) may be driven to produce higheroutput in order to compensate and maintain the overall design outputpower without over-driving individual elements, which otherwise wouldsignificantly increase the chances of further emitter failures due tojunction or facet damage.

FIG. 9 depicts another embodiment of the present invention, which addsdetectors to monitor the output power of the laser emitters, allowingcontrol of the pump source output in response to control signals. FIG. 9illustrates several possible alternative locations for the detectors,which preferably take the form of photodiodes sensitive at the emissionwavelength of the laser array, e.g., silicon photodiodes which aresensitive at around 980 nm. Detectors in location 940 preferablyrepresent an array of silicon photodiodes, disposed near the back facetof the emitters of laser array 910, and aligned to receive radiationemitted from the back facet on a one-to one basis, wherein each emitteris uniquely associated with one photodiode. The detector array may bemonolithic or it may comprise individual photodiode chips. Thephotodiodes may be attached to the submount either prior or subsequentto the laser array, provided that appropriate care is taken to selectprocesses that do not interfere with those described above for themounting of diode laser array 910 and IO waveguide chip 915. Ifrequired, the submount may incorporate features such as turning mirrorslocated adjacent to the back facet of laser array 910 to redirect thelight emitted from the back facet perpendicular to the plane of themajor surface of the submount. In this case the photodiode array may bemounted over the turning mirrors (either face down or in a back-litconfiguration) to receive the light emitted from the back facet, andsecured in place without contact with any electrical interconnectiontraces disposed on the submount surface for activation of the laserarray 910 emitters. Electrical contact to the photodiode array may bemade by solder joints or wire bonding or conductive epoxy. The outputsignals from the photodiode array are preferably transferred toelectrical connection traces on the submount surface and from there maybe wirebonded to output pins in the external package for connection tothe control electronics and laser drivers 970, as indicated symbolicallyby the dashed line 944. Alternatively, the output signal may be directlywirebonded to traces in the external package. The actual optical outputpower in each channel may be calibrated with respect to the photodiodesignal using a power meter to measure the output power in each outputfiber, and the calibration constants stored in memory within the drivercircuitry.

The detectors may alternatively be disposed at location 950. Detectorsin location 950 preferably represent an arrangement whereby the opticalpower in each output channel of the pump source is monitored via astructure incorporated into IO chip 915. FIGS. 10 and 11 show anembodiment of an integrated power monitoring array incorporated into thepresent invention. In this embodiment the power in each channel ismonitored by a photodiode 1052 disposed to receive light scattered outof optical waveguide 1080 by a distributed Bragg reflection (feedback)grating 1056. Alternatively, a waveguide discontinuity, pit or othersuitable scattering structure which extends into the evanescent tail ofthe waveguide mode may be substituted for grating 1056. Some scatteringis inherent in the practical implementation of a surface etched reliefgrating (as described in an embodiment above), even for a first ordergrating. The proportion of the input beam that is scattered out fromwaveguide 1080 is dependent on the mode intensity profile that interactswith the grating features. As the mode intensity profile does not changewith power, the scattering proportion should be constant, independent ofinput power. Thus a measure of the scattered light may be used as anindicator of the power incident (and transmitted through) the grating.Alternatively, the grating structure may be designed specifically toprovide an out of plane diffracted beam, for instance as a second orthird order grating, as is known in the art.

The photodiodes, which may be discrete devices or a monolithic array,are disposed to receive the light scattered or diffracted from thewaveguide by the grating. They may for instance be bonded face down tothe backside of the integrated optic chip at 1152, located substantiallyover the grating regions on the front face, as shown symbolically inFIG. 11. Alternatively the photodiodes may be located at 1153, recessedat least partially in wells 1110 in the submount 1105. These wells maybe fabricated by lithographic patterning and etching during the submountfabrication process. The detectors (or detector array) may be mountedinto the well using solder or conductive epoxy and may be either frontilluminated or back lit. Alternatively the photodiodes may be mounteddirectly to the front face of the integrated optics chip. The electricalsignals from the photodiodes may be carried by conductive traces definedover the surface of the submount to transfer the signals to the edge ofthe submount for connection to output pins in the external package, andfrom there to the control electronics and laser drivers 970 identifiedin FIG. 9 as indicated symbolically by 954. The mechanical andelectrical attach processes must of course be compatible with theprocessing outlined earlier for the laser array and integrated opticchip attach.

FIG. 12 illustrates an alternate embodiment of an integrated powermonitoring array with photodiode power monitoring detectors 1252 locatedat the outputs in integrated optic directional coupler taps 1210. Thedirectional coupler taps are preferably designed to couple only a smallproportion (preferably around 2-4%) of the power out of the primarywaveguides. Couplers taps 1210 for this purpose may be designed in asimilar manner to those described above for wavelength multiplexing ofthe laser emitter outputs. If desired, a scattering site or out-of-planereflector may be fabricated at the end of the directional coupler, e.g.by laser ablation, to more efficiently deflect the light into thephotodiode, which may be located on or near either the back or frontsurface of integrated optics chip 1215. As described above withreference to FIG. 11, the photodiodes (or monolithic array ofphotodiodes) may be located at least partially within wells in thesubmount, under the front surface of the IO waveguide chip.

Detectors in location 960 in FIG. 9 may represent a number ofphotodiodes illuminated by light from an array of fiber-optic taps 980,each coupling a small percentage (typically 2-4%) of the light from arespective one of the fibers in the output fiber array. Such taps arecommercially available and widely used for monitoring of fiber opticsignals as their coupling ratio (or proportion of power removed from themain fiber) is constant with wavelength and power, and is largelyundisturbed by fluctuations in environmental conditions. The fiber tapsmay be located inside or outside the overall package of the laser arrayamplifier pump source and may be butt coupled directly to monitorphotodiodes either individually or as an array.

FIG. 13 depicts another embodiment, wherein further functionality isadded to the laser array pump source in the form of redundancy toprovide protection against failure of individual laser emitters. In thisembodiment supernumerary (extra) laser emitters 1314 are fabricated inthe laser array 1310. For example, if the laser array pump source isdesigned to have 8 output fibers 1322 accepting light from 8 regularlaser emitters 1312, an extra 2-4 laser emitters may be provided in thearray. These extra emitters are coupled to extra waveguides 1334 in IOchip 1315 simultaneously with the regular emitters 1312 as describedabove. The extra waveguides in the IO waveguide chip may be providedwith all the functionality of the regular waveguides 1332, e.g. DBR,monitor photodiode, integrated optic directional coupler tap, etc. Fromthe output of the IO waveguide chip the extra waveguides are coupledinto extra fibers 1324 located in the output optical fiber arraysimultaneously with the regular output waveguides and fibers asdescribed above.

In operation of the laser array pump source, the regular laser emittersare energized to provide optical output power in the regular outputfibers for transmission to the amplifier region. Should one of theregular laser emitters fail, for instance due to defect induced failureof the laser diode junction or catastrophic optical damage at the outputfacet, one of the extra emitters may be energized and the extra outputfiber coupled to the now energized extra emitter connected to theamplifier region in place of the output fiber coupled to the failedlaser emitter. The act of connection of the extra output fiber to theamplifier region may take the form of a fiber fusion splice or the useof fiber optic connectors as known in the art. Note that preferably thediode laser emitters in the laser array are separated by sufficientlateral distance such that a defect occurring in one emitter junction oron the output facet of one emitter does not substantially affect theoperation of adjacent emitters.

FIG. 14 illustrates a further improvement to the present inventionproviding the addition of a switch network or fabric 1425 in the outputfiber array 1420 to provide redundancy. This switch network enablesdynamic and remote reconfiguration of the output from the laser arrayemitters into the fibers in the output fiber array in response tocontrol signals. Thus, should a regular emitter 1412 on the laser diodearray 1410 fail during operation of the laser array pump source, theoutput from one of the extra emitters 1414 may be switched into theappropriate output fiber for transmission to the amplifier region. Theact of switching may preferably be accomplished using a control signalto activate a desired switch from a remote location, allowing computercontrol of the laser pump source. Such switching control may make use ofoutput channel power monitors to determine whether a particular laseremitter is operating and providing power to a particular amplifierregion, and the switching control signals may be provided directly bythe laser array pump source drive and control electronics. The switchingnetwork may be composed of components known in the art such asthermo-optic switches, opto-mechanical fiber switches ormicro-electro-mechanical (MEMs) switches. Redundancy structures andtechniques may also be employed during the manufacturing process todeselect or reroute inoperative or poorly performing channels andthereby increase device yields. See U.S. Pat. No. 6,049,641,incorporated by reference herein.

FIG. 15 shows an alternate embodiment of the present invention where anoptical switch fabric or network 1525 is integrated into a waveguidenetwork on an integrated optic chip 1515. The integrated switch networkin the integrated optic chip may be comprised of thermo-optic orelectro-optic switches as known in the art, such as total internalreflection switches or switched directional couplers, which respond to acontrol signal in order to switch light between predetermined paths.

FIG. 16 shows an alternate embodiment where protection against laseremitter failure is provided by a passive network 1600 coupled to theoutput fiber array 220 identified in FIG. 2. The passive network 1600preferably comprises a series of 50-50 directional couplers 1670 andpower splitters 1680 which share the power entering the input fibers1625 of the network, among the fibers in the output fiber array 1635.The passive power splitter and coupler network is preferably composed offused fiber type components that offer efficient power splitting withvery little excess loss which would otherwise decrease the availableoutput power from the pump source.

With the incorporation of the passive network, the laser array pumpsource output becomes substantially tolerant of the failure ofindividual emitters. The failure of a single emitter simply decreasesthe power in each output fiber 1635 by a small amount as a consequenceof the power sharing provided by the passive network, rather thanresulting in a total loss of power in a single output fiber whichresults from a single emitter failure in the absence of either thepassive network or another redundancy arrangement.

Note that in all of these redundancy and failure protection embodimentsthe phrases “extra emitters”, “extra waveguides” and “extra fibers” mayrefer to single elements, or if desired, sets of elements, such as thewavelength multiplexed elements of the embodiment described above. Thusif one emitter from the wavelength multiplexed set of emitters shouldfail, a complete new set of emitters may if desired, be energized andswitched to the appropriate output fiber.

FIG. 17 shows yet another embodiment of a pump source 1700 according tothe present invention, which provides for a different approach to theattachment of the output fiber array. A submount 1705 carrying a laserarray 1710 and an IO waveguide array chip 1715 is preferably fabricatedfrom single crystal silicon. Towards the output end of the submount arefabricated v-grooves 1740 as known in the art for alignment of opticalfibers 1720. Such v-grooves are fabricated using photolithographicexposure and patterning of a suitable mask material disposed on thesurface of the silicon submount wafer, e.g. silicon dioxide, followed bywet etching of the single crystal silicon. The orientation of thesilicon wafer is selected appropriately, for example 100/110. Theetching is typically performed using potassium hydroxide (KOH), whichprovides a selective etching capability such that the wet etchingprocess produces v-grooves 1740. The width of v-groove 1740 at thesurface of the wafer is very accurately defined by the width of thelithographically patterned mask. Consequently the position of an opticalfiber resting in v-groove 1740 is accurately defined in both the lateraland vertical dimensions via the lithographic patterning process combinedwith the selective or preferential etching performance of the singlecrystal material. Preferably the width of v-groove 1740 is defined suchthat the center of the core of optical fiber 1720 is positionedsubstantially at the same vertical position above the aforementionedreference surface as the optical waveguide output facet of the IOwaveguide array chip 1715.

The alignment and attachment process of the laser array pump source ismodified from that described above as follows. During the attachment ofthe laser array, laser array chip 1710 is accurately aligned relative toalignment marks 1735 defined on the surface of the submount, which arethemselves accurately located with respect to the etched v-grooves. Thealignment of laser array chip 1710 are performed accurately in thelateral and yaw angle dimensions such that the multiple laser emittersare substantially centered on the axis 1745 of respective v-grooves1740. The longitudinal alignment may be less precise as the length of IOchip 1715 and the longitudinal extent of v-grooves 1740 are preferablychosen such that there is some overlap of the v-groove 1740 and IO chip1715 at the output end 1717 of IO chip 1715.

In the preparation of IO chip 1715 it is important to ensure that theend faces of the chip are polished accurately perpendicular to the axesof waveguides in the lateral dimension. IO chip 1715 is aligned to thediode laser and attached to the submount substantially as describedabove. The yaw angle alignment is particularly important to ensure thatthe output waveguides of IO chip 1715 are substantially centered overthe axes of respective v-grooves 1740.

The output optical fibers may then be aligned and attached usingv-grooves 1740 fabricated in the output end of the submount. Severalpossible techniques exist for aligning fibers 1720. Each fiber 1720 maybe individually prepared either with a perpendicularly cleaved andanti-reflection coated end face or with a 4 or 8 degree angle cleave orpolish to suppress back reflections. The individual fibers 1720 may thenbe placed in respective v-grooves 1740 and adjusted for optimumlongitudinal position by monitoring the light coupled into the fiberfrom the activated laser emitter(s) using a photodiode or power meter.The fibers may then be mechanically attached to the submount using, forinstance, solder or UV or thermally cured epoxy. Alternatively, thefibers may be pre-fabricated into an array and aligned and assembled tothe submount in a single process step.

The use of the above described v-grooves 1740 integrated into submount1705 for alignment of optical fibers 1720 is complicated by the preciselateral and angular positioning accuracy required during the mounting ofthe first component (laser array 1710) to submount 1705 to ensure thatthe laser emitters are substantially aligned to the centers ofrespective v-grooves 1720. In addition, the length of IO chip 1715 ispotentially limited by the thermal expansion coefficient mismatchbetween the submount and IO chip materials. If the expansion mismatch issufficiently great and IO chip 1715 sufficiently long, the differentialexpansion generated during operation of pump source 1700 may be severeenough to fracture the mechanical bonds between submount 1705 and IOchip 1715.

FIGS. 18 a-c show another embodiment of a pump source 1800 according tothe present invention. This embodiment avoids the differential expansionproblems noted above in connection with the FIG. 17 embodiment andenables packaging of relatively long IO chips having a thermal expansioncoefficient which differs from that of the submount. A laser array 1810and IO waveguide chip 1815 are aligned and attached using a referencesurface defined by the array of standoffs 1830 on a first submount 1805.As shown in FIGS. 18 a and 18 b, a second and separate submount 1806 isused at the output end of IO waveguide chip 1815 to facilitate alignmentand attachment of an output optical fiber array 1820. The secondsubmount 1806 is aligned to IO waveguide chip 1815 usinglithographically defined alignment and/or fiducial marks. Secondsubmount 1806 may be attached using substantially the same techniques asdescribed previously for attaching the IO waveguide chip to the laserarray/submount sub-assembly. Second submount 1806 is provided with astandoff structure which may comprise a second array of standofffeatures 1836 defining a second reference surface to which IO waveguidechip 1815 is contacted and attached. The second submount is alsopreferably adapted with a set of relief slots 1840, the centers of whichare substantially aligned laterally with the centers of respectiveoutput waveguides in IO waveguide chip 1815 after the attachment processis complete. Relief slots 1840, which may for example be fabricated insingle crystal silicon using wet etching or deep RIE processing, arepreferably large enough to accept a standard single mode optical fibercladding without contact with the side walls or bottom of the slot.Relief slots 1840 are preferably defined such that they extend under theoutput end of IO waveguide chip 1815, as shown in FIG. 18 a.

The output fiber array 1820 is preferably fabricated using a singlepiece of v-groove silicon 1824, as shown in FIG. 18 b. Such an assemblymay for instance be fabricated by bonding the fibers 1822 into singlev-groove piece 1824, followed by the temporary attachment of a secondpiece of v-groove silicon 1826 (indicated in phantom) to provide a“sandwich” around the fibers to support them during a standard opticalpolishing process, as known in the art. After completion of polishing,which may be perpendicular or at some predefined angle to the opticalaxis of fibers 1822, anti-reflection coatings may be applied if desiredand the temporarily attached second piece of v-groove silicon 1826removed to leave the structure of FIG. 18 b. Preferably the v-groovesare fabricated such that their lithographically defined width places thefiber core the same vertical distance from the surface of the siliconv-groove wafer as the IO waveguide chip optical waveguide is locatedbeneath the surface of the IO waveguide chip.

The fiber assembly may then be mounted on a computer controlled stage asdescribed earlier and positioned such that the fibers 1822 face therelief slots 1840 fabricated in second submount 1806. The v-groovesilicon is then brought down into contact with the second referencesurface defined by second array of standoff features 1836 on secondsubmount 1806. Using the techniques described earlier, substantiallyuniform contact between the silicon v-groove material and the referencesurface is achieved, substantially aligning the axes IO waveguide andthe optical fiber in the vertical dimension. After optimization of thelongitudinal and lateral positions of the fiber array relative to IOwaveguide chip 1815, the two components may be mechanically attached,for example, using UV cured epoxy or solder to produce the assemblyshown in FIG. 18 c. Those skilled in the art will recognize that caremust be taken to ensure that the attachment methods used in subsequentsteps in the assembly process are compatible with prior steps; for,example, the melting temperature of a solder used in a subsequent stepshould be lower than that used in a prior step in order to preventundesired melting and re-flow of solder employed for attachment in theprior step.

FIGS. 19 a and 19 b depict another embodiment of a pump source 1900 inaccordance with the present invention, which utilizes a modification ofthe single sided silicon v-groove mounted fiber array depicted in FIGS.18 a-c. In contradistinction to the previous embodiment, wherein theoptical fibers are polished normal to their longitudinal axes (or at ashallow angle thereto in order to suppress back reflections), opticalfibers 1922 may be polished at a relatively sharp angle from twoopposing sides to form chisel shaped fiber ends 1918, with the chiselshape preferably substantially centered on and symmetric about thecenter of a core 1928 of optical fiber 1922 as shown in plan and sideviews in FIG. 19 a. Again, the polishing process may be facilitated bythe addition of a temporarily attached second silicon v-groove piece,which is subsequently removed after polishing. The lithographicallydefined width of the v-grooves is preferably chosen such that thecenters of the cores 1928 of optical fibers 1922 are accurately locatedat the same vertical distance from the surface of the v-groove siliconwafer as the laser array emitters are located beneath the major surfaceof the diode laser wafer. The separation of the v-grooves is chosen tomatch the lateral spacing of the emitters in laser array 1910.

Laser array 1910 is coupled directly to an array 1920 of chisel or lensended fibers 1922 assembled in the single-sided v-groove holder 1924,omitting the IO waveguide chip shown in the above embodiments whilestill providing simultaneous coupling of all the emitters in laser array1910 using a standoff structure 1930 defining a reference surface toensure accurate, passive, alignment in the critical vertical dimension.Submount 1905 may also be adapted with relief slots 1940 as describedabove in connection with FIGS. 18 a and 18 c. In the presently describedembodiment it is preferable that laser array 1910 is mounted using thetechniques described above, and accurately positioned such that theemitters are substantially centered with respect to relief slots 1940.In the longitudinal dimension, laser array 1910 is preferably located inclose proximity to the proximal end of the corresponding relief slot1940 such that the slot does not substantially undercut the area ofarray 1910 that would otherwise compromise the electrical, mechanicaland thermal properties of the bond between laser array 1910 and submount1905.

Optical alignment in this embodiment is preferably performed by mountingv-groove fiber array 1920 on a computer controlled micropositioner withfibers 1922 facing relief slots 1940 in submount 1905. V-groove fiberarray 1920 is brought substantially into uniform contact with thereference surface using the techniques described above. The lateral andlongitudinal positions of fiber array 1920 are then optimized tomaximize the power coupled from the energized laser emitters to opticalfibers 1922. After final alignment, mechanical attachment of fiber array1922 to submount 1905 may be achieved using solder, thermally or UVcured epoxy, or other suitable adhesive to yield pump source 1900, asshown in FIG. 19 b.

FIG. 21 is a block diagram depicting yet another embodiment of thepresent invention, which provides for pumping of multiple EDFAs from asingle laser array pump source of this invention. This can provideadvantages such as significant simplification in construction of opticalfiber systems where multiple EDFAs are required at a given physicallocation, and compact design for signal amplification without WDM, wheredesired.

A laser array pump source 2310 is optically connected by its outputfibers 2320 to EDFAs 2390. While connection by three output fibers tothree EDFAs is illustrated for purposes of clarity in the figure, it isapparent that such connection may be made to a greater or lesser numberof EDFAs as desired for different applications, for example to 8 EDFAs,by 8 output fibers, as limited by the number of outputs of pump source2310. Each EDFA 2390 is shown to have a signal input fiber 2330, anamplified signal output fiber 2340, a forward pumping port 2370 and abackward pumping port 2380. Pump output fibers 2320 are shown to beconnected to the forward pumping ports 2370, but alternatively, theconnection may be made to backward pumping ports 2380 (as indicated bydashed lines), instead. Various EDFAs are known in the art and areavailable from commercial suppliers, such as the PureGain™2500C opticalamplifier available from Corning Incorporated (Corning, N.Y.).

Other alternate, known pump connection schemes may be employed. Forexample, simultaneous forward and backward pumping may be implementedaccording to this embodiment by connecting two different output fibersof the laser array pump source to each EDFA, one to the forward and oneto the backward pumping port. A laser array pump source of thisinvention can provide such pumping to a number of EDFAs that is one halfthe number of its outputs.

As used herein, a given event is “responsive” to a predecessor event ifthe predecessor event influenced the given event. If there is anintervening processing element, step or time period, the given event canstill be “responsive” to the predecessor event. If the interveningprocessing element or step combines more than one event, the signaloutput of the processing element or step is considered “responsive” toeach of the event inputs. If the given event is the same as thepredecessor event, this is merely a degenerate case in which the givenevent is still considered to be “responsive” to the predecessor event.“Dependency” of a given event upon another event is defined similarly.

It will be recognized by those skilled in the art that, while theinvention has been described above in terms of preferred embodiments, itis not limited thereto. For example, any and all variations described,suggested or incorporated by reference in the Background section of thispatent application are specifically incorporated by reference into thedescription herein of embodiments of the invention. In addition, variousfeatures and aspects of the above described invention may be usedindividually or jointly. Further, although the invention has beendescribed in the context of its implementation in a particularenvironment and for particular applications, e.g., telecommunications,those skilled in the art will recognize that its usefulness is notlimited thereto and that the present invention can be beneficiallyutilized in any number of environments and implementations. Accordingly,the claims set forth below should be construed in view of the fullbreadth and spirit of the invention as disclosed herein.

1. Optical apparatus comprising: a first submount having a standoffstructure protruding from a first surface thereof; an optical emitterchip bonded to said first submount, said optical emitter chip having afirst optical emitter and contacting said standoff structure in a firstplurality of contact portions of said standoff structure, said firstplurality of contact portions including all points on said standoffstructure which contact said emitter chip, at least three consecutiveones of said contact portions along a straight line being mutuallyisolated from each other along said straight line; and an integratedoptics chip having a first optical receiver, said integrated optics chipbeing bonded to said first submount in such a way that said firstoptical receiver can receive optical energy emitted by said firstemitter.
 2. Apparatus according to claim 1, wherein said integratedoptics chip contacts said standoff structure in a second plurality ofcontact portions of said standoff structure, said second plurality ofcontact portions including all points on said standoff structure whichcontact said integrated optics chip, at least three consecutive ones ofsaid contact portions in said second plurality of contact portions alonga given straight line being mutually isolated from each other along saidgiven straight line.
 3. Apparatus according to claim 1, furthercomprising an epoxy bonding said emitter chip to said first submount,said epoxy being located between two of said contact portions on saidfirst submount and not on any of said contact portions.
 4. Apparatusaccording to claim 1, further comprising solder bonding said emitterchip to said first submount, said solder being located between two ofsaid contact portions on said first submount and not on any of saidcontact portions.
 5. Apparatus according to claim 1, further comprising:an electrical trace disposed on said first submount between said contactportions; and an electrical connection pad on said emitter chip; and anelectrically conductive material bonding said emitter chip to said firstsubmount and making electrical contact with both said electrical traceon said first submount and said electrical connection pad on saidemitter chip.
 6. Apparatus according to claim 1, further comprising: anelectrical trace disposed on said first submount between said contactportions; and an electrical connection pad on said integrated opticschip; and an electrically conductive material bonding said integratedoptics chip to said first submount and making electrical contact withboth said electrical trace on said first submount and said electricalconnection pad on said integrated optics chip.
 7. Apparatus according toclaim 6, wherein said electrically conductive material is a solder. 8.Apparatus according to claim 1, wherein said emitter chip includes aplurality of optical emitters arranged along a first edge of saidemitter chip, and wherein at least three consecutive ones of saidcontact portions along a straight line parallel to said first edge ofsaid emitter chip are mutually isolated from each other along saidstraight line parallel to said first edge.
 9. Apparatus according toclaim 1, wherein said standoff structure comprises a plurality of ribsarranged such that each rib includes a respective first segment which isin said first plurality of contact portions and a respective secondsegment which extends under said integrated optics chip.
 10. Apparatusaccording to claim 1, wherein said integrated optics chip includes afirst optical output, further comprising a first optical fiber having areceiving end disposed such that it can receive optical energy outputfrom said first optical output of said integrated optics chip. 11.Apparatus according to claim 10, further comprising an optical amplifierhaving an optical pump input connected to receive optical energy fromsaid first optical fiber.
 12. Apparatus according to claim 10, whereinsaid integrated optics chip comprises a mode converter in an opticalpath from said first optical receiver to said first optical output. 13.Apparatus according to claim 10, wherein said integrated optics chipcomprises: an input waveguide segment receiving optical energy from saidfirst optical receiver and supporting an input optical mode; an outputwaveguide segment downstream of said input waveguide segment andproviding optical energy to said first optical output and supporting anoutput optical mode; and a mode converter disposed in an optical pathfrom said input optical waveguide segment to said output waveguidesegment and converting said input optical mode to said output opticalmode.
 14. Apparatus according to claim 13, wherein said input opticalmode optimizes acceptance of optical energy from said first emitter intosaid input waveguide segment.
 15. Apparatus according to claim 13,wherein said output optical mode optimizes delivery of optical energyfrom said first output of said integrated optics chip into said firstoptical fiber.
 16. Apparatus according to claim 13, wherein said inputoptical mode optimizes acceptance of optical energy from said firstemitter into said input waveguide segment, and wherein said outputoptical mode matches an optical mode supported by said first opticalfiber.
 17. Apparatus according to claim 13, wherein said mode convertercomprises a waveguide taper.
 18. Apparatus according to claim 1, whereinsaid integrated optics chip comprises: a first waveguide segment in afirst optical path from said first optical receiver to a first opticaloutput of said integrated optics chip; a first feedback gratingsuperimposed on said first waveguide segment; and a first opticalmonitor disposed to receive optical energy scattered out of saidwaveguide by said feedback grating.
 19. Apparatus according to claim 1,wherein said integrated optics chip comprises: a first waveguide segmentin a first optical path from said first optical receiver to a firstoptical output; and a first wavelength stabilizer disposed in said firstwaveguide segment.
 20. Apparatus according to claim 19, wherein saidwavelength stabilizer comprises a first feedback grating superimposed onsaid first waveguide segment.
 21. Apparatus according to claim 19,wherein said integrated optics chip further comprises a first waveguidetaper in said first path upstream of said first waveguide segment. 22.Apparatus according to claim 19, wherein said integrated optics chipfurther comprises a second waveguide taper in said first path downstreamof said first waveguide segment.
 23. Apparatus according to claim 19,wherein said emitter chip further has a second optical emitter and saidintegrated optics chip has a second optical receiver, said integratedoptics chip being disposed further so that said second optical receivercan receive optical energy emitted by said second emitter, and whereinsaid integrated optics chip further comprises a second wavelengthstabilizer in a second waveguide segment in a second optical pathdownstream of said second optical receiver.
 24. Apparatus according toclaim 23, wherein said first and second wavelength stabilizers are tunedto different wavelengths, and wherein said integrated optics chipfurther comprises a directional coupler in said first path downstream ofsaid first stabilizer and coupling optical energy from said second pathdownstream of said second stabilizer into said first path toward saidfirst optical output.
 25. Apparatus according to claim 23, wherein saidfirst and second wavelength stabilizers are tuned to differentwavelengths, wherein said second optical path in said integrated opticschip terminates with a second optical output of said integrated opticschip, further comprising a fused fiber coupler having first and secondreceiving optical fibers, said first receiving fiber being disposed suchthat it can receive optical energy output from said first optical outputof said integrated optics chip, and said second receiving fiber beingdisposed such that it can receive optical energy output from said secondoptical output of said integrated optics chip.
 26. Apparatus accordingto claim 1, wherein said emitter chip further has a second opticalemitter and said integrated optics chip has a second optical receiver,said integrated optics chip being disposed further so that said secondoptical receiver can receive optical energy emitted by said secondemitter, and wherein said integrated optics chip further comprises adirectional coupler having a first input in a first optical pathdownstream of said first optical receiver, a second input in a secondoptical path downstream of said second optical receiver, and an outputin an optical path upstream of said first optical output.
 27. Apparatusaccording to claim 1, wherein said integrated optics chip comprises apolarization rotator disposed in a first optical path from said firstoptical receiver to a first optical output of said integrated opticschip.
 28. Apparatus according to claim 27, wherein said emitter chipfurther has a second optical emitter and said integrated optics chip hasa second optical receiver, said integrated optics chip being disposedfurther so that said second optical receiver can receive optical energyemitted by said second emitter, and wherein said integrated optics chipfurther comprises a directional coupler in said first path downstream ofsaid polarization rotator and coupling optical energy into said firstpath toward said first optical output from said second optical receiver.29. Apparatus according to claim 1, wherein said integrated optics chipcomprises a power monitor disposed to monitor optical power received bysaid first optical receiver.
 30. Apparatus according to claim 1, whereinsaid integrated optics chip comprises a controllable optical switch. 31.Apparatus according to claim 1, wherein said emitter chip further has asecond and third optical emitters and said integrated optics chip hassecond and third optical receivers, said integrated optics chip beingdisposed further such that said second optical receiver of saidintegrated optics chip can receive optical energy emitted by said secondemitter and such that said third optical receiver of said integratedoptics chip can receive optical energy emitted by said third emitter,and wherein said integrated optics chip further comprises: a firstwaveguide in a first path from said first optical receiver to a firstoptical output of said integrated optical chip; a second waveguide in asecond path from said second optical receiver to a second optical outputof said integrated optical chip; and a cross-connect switching structurewhich switches optical energy from said third optical receiverselectably into (a) said first path toward said first optical output,(b) or said second path toward said second optical output, or (c)neither.
 32. Apparatus according to claim 1, wherein said integratedoptics chip has a plurality of optical outputs, further comprising anoptical fiber array having a plurality of optical fibers, each of saidfibers having a respective receiving end disposed to receive opticalenergy output from a respective one of the optical outputs of saidintegrated optics chip.
 33. Apparatus according to claim 32, whereinsaid optical fiber array is attached to said first submount. 34.Apparatus according to claim 33, wherein each of said optical fibers isaffixed in a respective longitudinally-oriented v-groove in said firstsubmount downstream of said integrated optics chip.
 35. Apparatusaccording to claim 33, wherein said first submount includeslongitudinally-oriented recesses, further comprising: a fiber holderhaving a plurality of longitudinally-oriented v-groove in theundersurface thereof, each of said fibers being affixed in a respectiveone of said v-grooves, and said fiber holder being attached to saidfirst submount with the undersurface of said fiber holder facing saidfirst surface of said first submount, said fibers depending below theundersurface of said fiber holder and into said recesses in said firstsubmount.
 36. Apparatus according to claim 32, wherein said integratedoptics chip has an overhang portion overhanging said first submount,further comprising: a second submount having a second standoff structureprotruding from a first surface thereof, said overhang portion of saidintegrated optics chip being attached to said second standoff structuresuch that said integrated optics chip contacts said second standoffstructure in a third plurality of contact portions of said secondstandoff structure, said optical fiber array being attached to saidsecond submount.
 37. Apparatus according to claim 36, wherein saidsecond submount is spaced longitudinally from said first submount. 38.Apparatus according to claim 36, wherein said second submount includeslongitudinally-oriented recesses, further comprising: a fiber holderhaving a plurality of longitudinally-oriented v-groove in theundersurface thereof, each of said fibers being affixed in a respectiveone of said v-grooves, and said fiber holder being attached to saidsecond submount with the undersurface of said fiber holder facing saidfirst surface of said second submount, said fibers depending below theundersurface of said fiber holder and into said recesses in said secondsubmount.
 39. Apparatus according to claim 32, further comprising anoptical amplifier having an optical pump input connected to receiveoptical energy from one of said optical fibers.
 40. Apparatus accordingto claim 32, further comprising a plurality of optical amplifiers eachhaving an optical pump input connected to receive optical energy from arespective one of said optical fibers.
 41. Apparatus according to claim1, wherein said integrated optics chip has first and second opticaloutputs, both having a common plane of optical polarization, furthercomprising: a polarization coupler having first and second polarizationmaintaining input fibers and an output fiber, said first input fiberbeing attached so as to receive optical energy from said first opticaloutput, said first input fiber being attached with a first rotationrelative to said output fiber, and said second input fiber beingattached so as to receive optical energy from said second opticaloutput, said second input fiber being attached with a second rotation90° different from said first input fiber.
 42. Optical apparatuscomprising: a submount having a standoff structure protruding from afirst surface thereof; an integrated optics chip bonded to saidsubmount, said integrated optics chip having a first optical receiverand contacting said standoff structure in a first plurality of contactportions of said standoff structure, said first plurality of contactportions including all points on said standoff structure which contactsaid integrated optics chip, at least three consecutive ones of saidcontact portions along a straight line being mutually isolated from eachother along said straight line; and an optical emitter chip having afirst optical emitter, said optical emitter chip being bonded to saidsubmount in such a way that said first optical receiver can receiveoptical energy emitted by said first emitter.
 43. Optical apparatuscomprising: a first submount having a standoff structure protruding froma first surface thereof; an optical array emitter chip bonded to saidfirst submount, said optical emitter chip having a plurality of opticalemitters and contacting said standoff structure in a first plurality ofcontact portions of said standoff structure, said first plurality ofcontact portions including all points on said standoff structure whichcontact said emitter chip, at least three consecutive ones of saidcontact portions along a straight line being mutually isolated from eachother along said straight line; and an optical fiber array having aplurality of optical fibers, said optical fiber array being bonded tosaid first submount in such a way that said a receiving end of each ofsaid fibers can receive optical energy emitted by a respective one ofsaid optical emitters.
 44. Apparatus according to claim 43, wherein saidoptical fiber array contacts said standoff structure in a secondplurality of contact portions of said standoff structure, said secondplurality of contact portions including all points on said standoffstructure which contact said optical fiber array, at least threeconsecutive ones of said contact portions in said second plurality ofcontact portions along a given straight line being mutually isolatedfrom each other along said given straight line.
 45. Apparatus accordingto claim 43, further comprising: an electrical trace disposed on saidfirst submount between said contact portions; and an electricalconnection pad on said emitter chip; and an electrically conductivematerial bonding said emitter chip to said first submount and makingelectrical contact with both said electrical trace on said firstsubmount and said electrical connection pad on said emitter chip. 46.Apparatus according to claim 43, wherein said optical emitters arearranged along a first edge of said emitter chip, and wherein at leastthree consecutive ones of said contact portions along a straight lineparallel to said first edge of said emitter chip are mutually isolatedfrom each other along said straight line parallel to said first edge.47. Apparatus according to claim 43, wherein said standoff structurecomprises a plurality of ribs arranged such that each rib includes arespective first segment which is in said first plurality of contactportions and a respective second segment which extends under saidoptical fiber array.
 48. Apparatus according to claim 43, wherein saidfirst submount includes longitudinally-oriented recesses, wherein saidoptical fiber array comprises a fiber holder having a plurality oflongitudinally-oriented v-groove in the undersurface thereof, each ofsaid fibers being affixed in a respective one of said v-grooves, and andwherein said fiber holder is attached to said first submount with theundersurface of said fiber holder facing said first surface of saidfirst submount, said fibers depending below the undersurface of saidfiber holder and into said recesses in said first submount. 49.Apparatus according to claim 43, further comprising an optical amplifierhaving an optical pump input connected to receive optical energy fromone of said optical fibers.
 50. Apparatus according to claim 43, furthercomprising a plurality of optical amplifiers each having an optical pumpinput connected to receive optical energy from a respective one of saidoptical fibers.
 51. Optical apparatus comprising: a submount having astandoff structure protruding from a first surface thereof; anintegrated optics chip bonded to said submount, said integrated opticschip having a first optical receiver and contacting said standoffstructure in a first plurality of contact portions of said standoffstructure, said first plurality of contact portions including all pointson said standoff structure which contact said integrated optics chip, atleast three consecutive ones of said contact portions along a straightline being mutually isolated from each other along said straight line;and an optical emitter chip having a first optical emitter, said opticalemitter chip being bonded to said submount in such a way that said firstoptical receiver can receive optical energy emitted by said firstemitter.